From e4871b12fcf3ce49c3fcaeb487eb2a79d13afd5f Mon Sep 17 00:00:00 2001
From: Neil <4138956+nwparker@users.noreply.github.com>
Date: Fri, 4 Sep 2026 00:11:21 -0700
Subject: [PATCH] fix(ssh): move Windows file writes off PowerShell 5.1 stdin
 onto sftp
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

#16432 was fixed by chunking writes to 32KB, on the belief that a
`DefaultShell=cmd.exe` host caps one stdin at roughly 50KB. Re-measured on
Windows 11 26200.9168 / OpenSSH_for_Windows_10.0p2, that premise is wrong in
both directions, and the chunking does not fix the hang.

The real constraint: a read on Windows PowerShell 5.1's redirected-stdin handle
over a non-pty ssh exec can die permanently when it finds the stream momentarily
empty, taking both the remaining data and the EOF with it. It is probabilistic
per such read — not a size threshold, and not certain on the first one. Measured
by swapping the copy loop for a counting reader:

  a 1.5s gap before any byte    -> 0 bytes received, 6 of 6
  1 byte, 1.5s gap, then 32767  -> exactly 1 byte
  32768, 1.5s gap, then 32768   -> exactly 32768
  a continuous 2MB              -> 167936 / 270336 / 372736

Those three 2MB figures are one payload run three times under the same
conditions, which is what rules out a threshold. Independently reproduced by a
second harness where one 1.9MB counted read completed through 39 reads and
another died after 11.

A payload that fits one burst usually presents only one read that can find the
stream empty, which is why 32KB mostly works — and it still failed 15 times in
120 under load, and 1 in 40 on a quiet host. Neither rate survives the 62 execs
a 1.9MB file needs: even 2.5% compounds to about four uploads in five failing.
No chunk size helps, because the defect is per blocking read, not per byte.
Three controls on the same host, same DefaultShell, rule out both a size limit
and cmd.exe: `findstr` took 2,016,000 bytes through one exec's stdin, sftp moved
1.9MB 5/5, and PowerShell 7 took 2MB in one exec.

Windows writes now go over the sftp subsystem, whose batch script is read by
the *local* client, so no remote process reads a pipe at all. PowerShell 7 is
the fallback where sftp is unavailable, and Windows PowerShell 5.1 is last,
still bounded, and now reports the host limitation and its remedy instead of a
bare timeout.

Measured on the same host, through this code: 1.9MB x20 all succeeded,
hash-verified, median 315ms, against 0/6 before. 32KB x120 zero hangs, against
15/120.

Also:
- Stage under a unique name per attempt. An abandoned write leaves a remote
  process that may still hold the staging file, and losing contact is not
  evidence it died (docs/reference/ssh-execution-boundary.md), so a retry must
  not reuse a name its predecessor may own. Sweep is best-effort and never
  treated as proof of anything.
- Create upload directories over sftp too; the JSON mkdir batch rode the same
  defective read.
- Cover makeWindowsWriteFileCommand and the publish command against the
  8000-char budget, which F11 flagged as untested.
---
 src/main/ssh/ssh-remote-powershell.ts         |  20 +-
 ...-remote-windows-command-line-limit.test.ts |  44 ++
 src/main/ssh/ssh-system-fallback.test.ts      |  83 +--
 .../ssh/system-ssh-file-binary-transfer.ts    | 236 +++-----
 src/main/ssh/system-ssh-file-transfer.ts      |  56 +-
 src/main/ssh/system-ssh-sftp-args.test.ts     | 100 ++++
 src/main/ssh/system-ssh-sftp-args.ts          |  84 +++
 src/main/ssh/system-ssh-sftp-path.test.ts     |  59 ++
 src/main/ssh/system-ssh-sftp-path.ts          |  46 ++
 src/main/ssh/system-ssh-sftp-transfer.ts      | 176 ++++++
 src/main/ssh/system-ssh-windows-file-write.ts | 121 ++++
 .../ssh/system-ssh-windows-upload.test.ts     | 555 +++++++++++++-----
 ...tem-ssh-windows-write-capabilities.test.ts |  79 +++
 .../system-ssh-windows-write-capabilities.ts  |  52 ++
 .../ssh/system-ssh-windows-write-strategy.ts  | 301 ++++++++++
 15 files changed, 1660 insertions(+), 352 deletions(-)
 create mode 100644 src/main/ssh/system-ssh-sftp-args.test.ts
 create mode 100644 src/main/ssh/system-ssh-sftp-args.ts
 create mode 100644 src/main/ssh/system-ssh-sftp-path.test.ts
 create mode 100644 src/main/ssh/system-ssh-sftp-path.ts
 create mode 100644 src/main/ssh/system-ssh-sftp-transfer.ts
 create mode 100644 src/main/ssh/system-ssh-windows-file-write.ts
 create mode 100644 src/main/ssh/system-ssh-windows-write-capabilities.test.ts
 create mode 100644 src/main/ssh/system-ssh-windows-write-capabilities.ts
 create mode 100644 src/main/ssh/system-ssh-windows-write-strategy.ts

diff --git a/src/main/ssh/ssh-remote-powershell.ts b/src/main/ssh/ssh-remote-powershell.ts
index 8c94fd3c483..420223ced29 100644
--- a/src/main/ssh/ssh-remote-powershell.ts
+++ b/src/main/ssh/ssh-remote-powershell.ts
@@ -11,14 +11,24 @@ export {
 // to leave room for the `/c` wrapper sshd adds before cmd.exe counts the line.
 const WINDOWS_REMOTE_COMMAND_LINE_BUDGET_CHARS = 8_000
 
-export function powerShellCommand(script: string): string {
-  const inline = encodedPowerShellCommand(script)
+/**
+ * `pwsh.exe` is PowerShell 7. It is not present on a stock Windows install, so it is only ever
+ * chosen after a probe — but where it exists it reads a redirected stdin correctly, which Windows
+ * PowerShell 5.1 does not (see `system-ssh-file-binary-transfer.ts`).
+ */
+export type WindowsPowerShellExecutable = 'powershell.exe' | 'pwsh.exe'
+
+export function powerShellCommand(
+  script: string,
+  executable: WindowsPowerShellExecutable = 'powershell.exe'
+): string {
+  const inline = encodedPowerShellCommand(script, executable)
   if (inline.length <= WINDOWS_REMOTE_COMMAND_LINE_BUDGET_CHARS) {
     return inline
   }
   // Why: these scripts are repetitive enough that gzip beats the UTF-16LE tax by
   // ~4x, which is the difference between a line cmd.exe runs and one it refuses.
-  const compressed = encodedPowerShellCommand(selfExtractingPowerShellScript(script))
+  const compressed = encodedPowerShellCommand(selfExtractingPowerShellScript(script), executable)
   if (compressed.length > WINDOWS_REMOTE_COMMAND_LINE_BUDGET_CHARS) {
     throw new Error(
       `Remote Windows command needs ${compressed.length} characters; Orca budgets ${WINDOWS_REMOTE_COMMAND_LINE_BUDGET_CHARS} for a line sshd hands to cmd.exe, which itself refuses more than ${CMD_EXE_COMMAND_LINE_MAX_CHARS}.`
@@ -27,8 +37,8 @@ export function powerShellCommand(script: string): string {
   return compressed
 }
 
-function encodedPowerShellCommand(script: string): string {
-  return `powershell.exe -NoProfile -NonInteractive -ExecutionPolicy Bypass -EncodedCommand ${encodePowerShellCommand(script)}`
+function encodedPowerShellCommand(script: string, executable: WindowsPowerShellExecutable): string {
+  return `${executable} -NoProfile -NonInteractive -ExecutionPolicy Bypass -EncodedCommand ${encodePowerShellCommand(script)}`
 }
 
 /** Orca-prefixed names so the payload can never shadow the bootstrap's own state. */
diff --git a/src/main/ssh/ssh-remote-windows-command-line-limit.test.ts b/src/main/ssh/ssh-remote-windows-command-line-limit.test.ts
index c104078238e..fa34a8fbda7 100644
--- a/src/main/ssh/ssh-remote-windows-command-line-limit.test.ts
+++ b/src/main/ssh/ssh-remote-windows-command-line-limit.test.ts
@@ -4,6 +4,10 @@ import { CMD_EXE_COMMAND_LINE_MAX_CHARS } from '../providers/windows-shell-args'
 import { getRemoteHostPlatform } from './ssh-remote-platform'
 import { tryStealInstallLockCommand } from './ssh-relay-install-lock-commands'
 import { decodeRemotePowerShellScript, powerShellCommand } from './ssh-remote-powershell'
+import {
+  makeWindowsPublishStagedFileCommand,
+  makeWindowsWriteFileCommand
+} from './system-ssh-windows-file-write'
 import {
   cleanupOwnedRelayUploadStageCommand,
   promoteOwnedRelayUploadStageCommand,
@@ -38,6 +42,17 @@ describe('Windows remote command line limit', () => {
     [
       'steal stale install lock',
       tryStealInstallLockCommand(windows, 'C:\\Users\\orca\\.orca-remote\\relay', 1_200)
+    ],
+    // F11 flagged these two as uncovered. They carry one path literal each, so they are the file
+    // commands whose length a caller can actually move.
+    ['write file', makeWindowsWriteFileCommand('C:\\Users\\orca\\.orca-remote\\relay.js')],
+    [
+      'publish staged file',
+      makeWindowsPublishStagedFileCommand(
+        'C:\\Users\\orca\\.orca-remote\\relay.js.orca-partial-0123456789ab',
+        'C:\\Users\\orca\\.orca-remote\\relay.js',
+        'create'
+      )
     ]
   ])('keeps the %s command inside what sshd\u2019s cmd.exe accepts', (_name, command) => {
     expect(command.length).toBeLessThanOrEqual(CMD_EXE_COMMAND_LINE_MAX_CHARS)
@@ -76,3 +91,32 @@ describe('Windows remote command line limit', () => {
     )
   })
 })
+
+/**
+ * F11 asked whether a pathological path could reach the budget, and what happens if it does.
+ * Measured: the inline encoding crosses 8000 at roughly 2500 high-entropy path characters — an
+ * order of magnitude past what Windows itself accepts — and the failure is a throw before any ssh
+ * is spawned, never a hang.
+ */
+describe('Windows file command budget headroom', () => {
+  it('absorbs a path far longer than Windows will accept', () => {
+    const deep = `C:\\Users\\orca\\${'segment\\'.repeat(30)}relay.js`
+
+    expect(deep.length).toBeGreaterThan(260)
+    expect(makeWindowsWriteFileCommand(deep).length).toBeLessThanOrEqual(
+      CMD_EXE_COMMAND_LINE_MAX_CHARS
+    )
+  })
+
+  it('throws rather than spawning a line cmd.exe would refuse', () => {
+    // Random segments so gzip cannot rescue it, which is the only way to reach the ceiling at all.
+    const incompressible = Array.from(
+      { length: 400 },
+      (_unused, index) => `${index}-${Math.random().toString(36).slice(2)}`
+    ).join('\\')
+
+    expect(() => makeWindowsWriteFileCommand(`C:\\${incompressible}\\f.bin`)).toThrow(
+      /Orca budgets 8000/
+    )
+  })
+})
diff --git a/src/main/ssh/ssh-system-fallback.test.ts b/src/main/ssh/ssh-system-fallback.test.ts
index c366e899bf6..2a80ab031cf 100644
--- a/src/main/ssh/ssh-system-fallback.test.ts
+++ b/src/main/ssh/ssh-system-fallback.test.ts
@@ -709,9 +709,13 @@ describe('spawnSystemSsh', () => {
     expect(args[standaloneControlIdx + 1]).toBe('none')
   })
 
-  it('writes files to Windows system SSH targets with PowerShell stdin bytes', async () => {
-    const proc = createEventedProcess()
-    spawnMock.mockImplementation(() => closeOnceSpawned(proc))
+  it('sends Windows file writes over sftp, not through a remote PowerShell stdin', async () => {
+    const spawned: EventedProcess[] = []
+    spawnMock.mockImplementation(() => {
+      const proc = createEventedProcess()
+      spawned.push(proc)
+      return closeOnceSpawned(proc)
+    })
     const hostPlatform = getRemoteHostPlatform('win32-x64')
 
     const promise = writeFileViaSystemSsh(
@@ -722,16 +726,22 @@ describe('spawnSystemSsh', () => {
     )
 
     await expect(promise).resolves.toBeUndefined()
-    const args = spawnMock.mock.calls[0][1] as string[]
-    const remoteCommand = args.at(-1) ?? ''
-    expect(remoteCommand).toContain('powershell.exe')
-    expect(remoteCommand).not.toContain('/bin/sh')
-    expect(proc.stdin.end).toHaveBeenCalledWith(Buffer.from('0.1.0', 'utf-8'))
+    // #16432, re-measured: Windows PowerShell 5.1 can lose a redirected stdin for good when a read
+    // finds it momentarily empty, so the bytes must not travel that way at all.
+    const batch = String(spawned[0]!.stdin.end.mock.calls[0]?.[0] ?? '')
+    expect(batch).toContain('put ')
+    expect(batch).toContain('/C:/Users/me/.orca-remote/relay/.version.orca-partial-')
+    const sftpArgs = spawnMock.mock.calls[0][1] as string[]
+    expect(sftpArgs).toContain('-b')
+    // The rename that publishes it reads the staged file, never a pipe.
+    const publish = (spawnMock.mock.calls[1][1] as string[]).at(-1) ?? ''
+    expect(publish).toContain('powershell.exe')
+    expect(decodePowerShellCommand(publish)).toContain('[System.IO.File]::Move($staging, $path)')
+    expect(publish).not.toContain('/bin/sh')
   })
 
-  it('writes binary buffers to Windows system SSH targets with CreateNew mode', async () => {
-    const proc = createEventedProcess()
-    spawnMock.mockImplementation(() => closeOnceSpawned(proc))
+  it('enforces an exclusive Windows buffer write at the rename, where it is atomic', async () => {
+    spawnMock.mockImplementation(() => closeOnceSpawned(createEventedProcess()))
     const hostPlatform = getRemoteHostPlatform('win32-x64')
 
     const promise = writeBufferViaSystemSsh(
@@ -742,12 +752,11 @@ describe('spawnSystemSsh', () => {
     )
 
     await expect(promise).resolves.toBeUndefined()
-    const args = spawnMock.mock.calls[0][1] as string[]
-    const remoteCommand = args.at(-1) ?? ''
-    expect(remoteCommand).toContain('powershell.exe')
-    expect(decodePowerShellCommand(remoteCommand)).toContain('CreateNew')
-    expect(remoteCommand).not.toContain('/bin/sh')
-    expect(proc.stdin.end).toHaveBeenCalledWith(Buffer.from('png'))
+    const publish = decodePowerShellCommand((spawnMock.mock.calls[1][1] as string[]).at(-1) ?? '')
+    // `File::Move` raising on an existing destination is what carries the exclusive contract now;
+    // a `CreateNew` on the staged file would only refuse a leftover of our own.
+    expect(publish).toContain('[System.IO.File]::Move($staging, $path)')
+    expect(publish).not.toContain('[System.IO.File]::Delete($path)')
   })
 
   it('downloads files from Windows system SSH targets with PowerShell stdout bytes', async () => {
@@ -779,8 +788,7 @@ describe('spawnSystemSsh', () => {
   })
 
   it('forces standalone SSH for Windows file writes when requested', async () => {
-    const proc = createEventedProcess()
-    spawnMock.mockImplementation(() => closeOnceSpawned(proc))
+    spawnMock.mockImplementation(() => closeOnceSpawned(createEventedProcess()))
     const hostPlatform = getRemoteHostPlatform('win32-x64')
 
     const promise = writeFileViaSystemSsh(
@@ -791,10 +799,14 @@ describe('spawnSystemSsh', () => {
     )
 
     await expect(promise).resolves.toBeUndefined()
-    const args = spawnMock.mock.calls[0][1] as string[]
-    const standaloneControlIdx = args.indexOf('-S')
+    const sftpArgs = spawnMock.mock.calls[0][1] as string[]
+    // sftp's own `-S` names a program to run, so the same request has to be spelled as an option.
+    expect(sftpArgs).not.toContain('-S')
+    expect(sftpArgs).toContain('ControlPath=none')
+    const publishArgs = spawnMock.mock.calls[1][1] as string[]
+    const standaloneControlIdx = publishArgs.indexOf('-S')
     expect(standaloneControlIdx).toBeGreaterThan(-1)
-    expect(args[standaloneControlIdx + 1]).toBe('none')
+    expect(publishArgs[standaloneControlIdx + 1]).toBe('none')
   })
 
   it('uploads a Windows directory as a mkdir batch plus per-file writes, never one blob', async () => {
@@ -819,19 +831,20 @@ describe('spawnSystemSsh', () => {
       rmSync(localDir, { recursive: true, force: true })
     }
 
+    // #16432: directories first, then the file — but both over sftp now, so the only PowerShell
+    // left is the rename that publishes the staged file, which reads a file rather than a pipe.
+    const mkdirBatch = String(spawned[0]!.stdin.end.mock.calls[0]?.[0] ?? '')
+    expect(mkdirBatch).toBe('-mkdir "/C:/Users/me/.orca-remote/relay"\n')
+    const putBatch = String(spawned[1]!.stdin.end.mock.calls[0]?.[0] ?? '')
+    expect(putBatch).toContain('put ')
+    expect(putBatch).toContain('/C:/Users/me/.orca-remote/relay/relay.js.orca-partial-')
     const commands = spawnMock.mock.calls.map((call) => (call[1] as string[]).at(-1) ?? '')
-    // #16432: directories first (metadata only), then the file bytes on their own stdin. One batch
-    // meant base64-ing the whole bundle into a single PowerShell string, which the remote never read.
-    expect(commands).toHaveLength(2)
-    expect(commands.every((command) => command.includes('powershell.exe'))).toBe(true)
     expect(commands.every((command) => !command.includes('/bin/sh'))).toBe(true)
     expect(commands.join('\n')).not.toContain('tar -xzf')
-    expect(JSON.parse(spawned[0].stdin.end.mock.calls[0]?.[0] as string)).toEqual([
-      'C:/Users/me/.orca-remote/relay'
-    ])
-    expect(Buffer.from(spawned[1].stdin.end.mock.calls[0]?.[0] as Buffer).toString('utf-8')).toBe(
-      'console.log("relay")'
-    )
+    // Nothing base64s the bundle into one PowerShell string any more, and nothing reads one.
+    expect(
+      commands.some((command) => decodePowerShellCommand(command).includes('OpenStandardInput'))
+    ).toBe(false)
   })
 
   it('forces standalone SSH for Windows upload packages when requested', async () => {
@@ -855,9 +868,9 @@ describe('spawnSystemSsh', () => {
     }
 
     const args = spawnMock.mock.calls[0][1] as string[]
-    const standaloneControlIdx = args.indexOf('-S')
-    expect(standaloneControlIdx).toBeGreaterThan(-1)
-    expect(args[standaloneControlIdx + 1]).toBe('none')
+    // The first spawn is the sftp client, whose own `-S` names a program to run.
+    expect(args).not.toContain('-S')
+    expect(args).toContain('ControlPath=none')
   })
 
   it('throws when no system ssh is found', () => {
diff --git a/src/main/ssh/system-ssh-file-binary-transfer.ts b/src/main/ssh/system-ssh-file-binary-transfer.ts
index b0c5b662ed1..149d10dfbd3 100644
--- a/src/main/ssh/system-ssh-file-binary-transfer.ts
+++ b/src/main/ssh/system-ssh-file-binary-transfer.ts
@@ -1,5 +1,7 @@
 import { constants, createWriteStream } from 'node:fs'
-import { lstat, open } from 'node:fs/promises'
+import { lstat, mkdtemp, open, rm, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
 import type { Writable } from 'node:stream'
 import { pipeline } from 'node:stream/promises'
 import type { SshTarget } from '../../shared/ssh-types'
@@ -16,6 +18,16 @@ import {
   throwIfAborted,
   waitForChannelClose
 } from './system-ssh-operation-lifecycle'
+import {
+  writeWindowsRemoteFile,
+  type WindowsWriteSource
+} from './system-ssh-windows-write-strategy'
+
+export {
+  WINDOWS_STDIN_WRITE_CHUNK_BYTES,
+  WINDOWS_STDIN_WRITE_TIMEOUT_MS
+} from './system-ssh-windows-write-strategy'
+export { WINDOWS_STAGED_WRITE_SUFFIX } from './system-ssh-windows-file-write'
 
 type SystemSshOperationOptions = SystemSshBuildArgsOptions & {
   signal?: AbortSignal
@@ -74,13 +86,16 @@ export async function writeBufferViaSystemSsh(
 ): Promise<void> {
   throwIfAborted(options?.signal)
   if (options?.hostPlatform && isWindowsRemoteHost(options.hostPlatform)) {
-    await writeWindowsBytesViaSystemSsh(
+    await writeWindowsRemoteFile(
       target,
       remotePath,
-      contents.length,
-      (offset, maxBytes) =>
-        Promise.resolve(contents.subarray(offset, Math.min(offset + maxBytes, contents.length))),
-      options
+      {
+        totalBytes: contents.length,
+        readChunk: (offset, maxBytes) =>
+          Promise.resolve(contents.subarray(offset, Math.min(offset + maxBytes, contents.length))),
+        withLocalFile: (send) => withTemporaryLocalFile(contents, send)
+      },
+      options ?? {}
     )
     return
   }
@@ -127,20 +142,19 @@ export async function uploadFileViaSystemSsh(
     throwIfAborted(options?.signal)
 
     if (options?.hostPlatform && isWindowsRemoteHost(options.hostPlatform)) {
-      // #16432: a Windows host cannot take a whole file through one stdin, however the local side
-      // paces it — see WINDOWS_STDIN_WRITE_CHUNK_BYTES. This is the path that carries the large
-      // files, so it is the one that has to be chunked and bounded.
-      await writeWindowsBytesViaSystemSsh(
-        target,
-        remotePath,
-        openedStat.size,
-        async (offset, maxBytes) => {
+      // This is the path that carries the large files, so it is the one the transport choice is
+      // made for; see the #16432 note below.
+      const source: WindowsWriteSource = {
+        totalBytes: openedStat.size,
+        readChunk: async (offset, maxBytes) => {
           const buffer = Buffer.allocUnsafe(Math.min(maxBytes, openedStat.size - offset))
           const { bytesRead } = await handle.read(buffer, 0, buffer.length, offset)
           return buffer.subarray(0, bytesRead)
         },
-        options
-      )
+        // The verified local file is already exactly the payload, so sftp sends it as is.
+        withLocalFile: (send) => send(localPath)
+      }
+      await writeWindowsRemoteFile(target, remotePath, source, options ?? {})
       return
     }
 
@@ -173,158 +187,56 @@ export async function uploadFileViaSystemSsh(
 }
 
 /**
- * #16432: Windows PowerShell 5.1 stops draining a redirected stdin over a non-pty ssh exec
- * somewhere between 50KB and 1MB, depending on the host's `DefaultShell`, and it hangs rather than
- * failing. The reporter measured that on both constructs he tried — `[Console]::In.ReadToEnd()` and
- * `new IO.StreamReader([Console]::OpenStandardInput())`, the latter reading incrementally, which is
- * why the limit cannot be attributed to materializing the payload. `Stream.CopyTo` reads the same
- * `[Console]::OpenStandardInput()` object with the same incremental `Read` loop, so nothing in it
- * escapes that limit either: no single write may exceed what one stdin is known to carry.
+ * #16432, re-measured: the constraint is not a size limit, and it is not cmd.exe's.
  *
- * 32KB is an order of magnitude under the low end of the measured range, and under 50KB, which the
- * reporter measured succeeding against a stream reader on the worse of the two `DefaultShell`
- * settings.
- */
-export const WINDOWS_STDIN_WRITE_CHUNK_BYTES = 32 * 1024
-
-/** No Windows stdin write should ever outlive this; a wedged PowerShell never closes on its own. */
-export const WINDOWS_STDIN_WRITE_TIMEOUT_MS = 60_000
-
-/** Suffix for the path a multi-exec Windows write lands on before it is published by rename. */
-export const WINDOWS_STAGED_WRITE_SUFFIX = '.orca-partial'
-
-/**
- * Splits one logical Windows write into stdin-sized execs.
+ * A read on Windows PowerShell 5.1's redirected-stdin handle over a non-pty ssh exec can die
+ * permanently when it finds the stream momentarily empty: no further bytes arrive, and no EOF ever
+ * does. It is probabilistic per such read — not a size threshold, and not certain on the first one.
+ * Measured on Windows 11 26200.9168 / OpenSSH_for_Windows_10.0p2 with `DefaultShell = cmd.exe`, by
+ * replacing the copy loop with a counting reader:
  *
- * A write that needs more than one exec cannot land on the destination directly: a chunk failing
- * mid-file would leave a truncated artifact under the real name with nothing marking it incomplete,
- * and the retry would then meet its own leftovers — under `exclusive` the retry's `CreateNew` fails
- * on them. Multi-exec creates therefore land on a staging path and are published by a rename, which
- * is also where `exclusive` is enforced: once, at the destination, instead of smeared across the
- * first chunk. A caller-requested append cannot be staged without reading the remote file back, so
- * it keeps writing straight through, as its own protocol already implies.
+ *   - a 1.5s gap before any byte, which forces the first read to find nothing -> 0 bytes, 6 of 6
+ *   - one byte, a 1.5s gap, then 32767 more  -> exactly 1 byte, then nothing
+ *   - 32768, a 1.5s gap, then 32768 more     -> exactly 32768, then nothing
+ *   - a continuous 2MB                       -> 167936 / 270336 / 372736, then nothing
+ *
+ * Those three 2MB death points are one payload run three times under the same conditions, which is
+ * what rules out a threshold: a stream that died at a fixed point would not vary by 2x. Independently reproduced by
+ * a second harness, where one 1.9MB counted read survived 39 reads to completion and another died
+ * after 11 — same construct, same payload.
+ *
+ * A payload small enough to arrive in one burst usually presents only one read that can find the
+ * stream empty (the one waiting for EOF), which is why 32KB mostly works: it still failed 15 times
+ * in 120 with the host under load, and 1 in 40 on a quiet one. Neither rate is survivable across
+ * the 62 execs a 1.9MB file needs — even 2.5% compounds to roughly four uploads in five failing —
+ * and no chunk size helps, because the client does not control whether its bytes arrive together.
+ *
+ * The same host, same `DefaultShell`, same connection pattern contradicts every size-limit reading:
+ * `findstr` took 2,016,000 bytes through one exec's stdin, and PowerShell 7 took 2MB. So cmd.exe is
+ * not the ceiling and neither is ~50KB. Writes now go over sftp, which moves the whole payload
+ * without any remote process reading a pipe; see `system-ssh-windows-write-strategy.ts` for the
+ * fallback order.
+ *
+ * Successes are never partial. Across every run in both harnesses a failed write hung; not one
+ * produced a short file, so this defect cannot silently truncate an upload.
  */
-async function writeWindowsBytesViaSystemSsh(
-  target: SshTarget,
-  remotePath: string,
-  totalBytes: number,
-  readChunk: (offset: number, maxBytes: number) => Promise<Buffer>,
-  options: SystemSshWriteBufferOptions
-): Promise<void> {
-  throwIfAborted(options.signal)
-  const staged = !options.append && totalBytes > WINDOWS_STDIN_WRITE_CHUNK_BYTES
-  const writePath = staged ? `${remotePath}${WINDOWS_STAGED_WRITE_SUFFIX}` : remotePath
-  let offset = 0
-  // An empty write still has to run: it is what creates (or truncates) the file.
-  do {
-    const chunk = await readChunk(offset, WINDOWS_STDIN_WRITE_CHUNK_BYTES)
-    if (chunk.length === 0 && offset < totalBytes) {
-      throw new Error(`Source ran short during upload of ${remotePath}`)
-    }
-    await writeWindowsChunkViaSystemSsh(
-      target,
-      writePath,
-      chunk,
-      {
-        ...options,
-        append: staged ? offset > 0 : options.append === true || offset > 0,
-        exclusive: staged ? false : options.exclusive === true && offset === 0
-      },
-      offset
-    )
-    offset += chunk.length
-  } while (offset < totalBytes)
-  if (staged) {
-    await publishWindowsStagedWrite(target, writePath, remotePath, options)
+
+/** A staged write is materialized locally first when the source is a buffer rather than a file. */
+async function withTemporaryLocalFile<T>(
+  contents: Buffer,
+  send: (localPath: string) => Promise<T>
+): Promise<T> {
+  const directory = await mkdtemp(join(tmpdir(), 'orca-win-upload-'))
+  const localPath = join(directory, 'payload.bin')
+  try {
+    // 0600: the payload can be repository content, and tmpdir is shared on every platform.
+    await writeFile(localPath, contents, { mode: 0o600 })
+    return await send(localPath)
+  } finally {
+    await rm(directory, { recursive: true, force: true }).catch(() => {})
   }
 }
 
-async function writeWindowsChunkViaSystemSsh(
-  target: SshTarget,
-  remotePath: string,
-  chunk: Buffer,
-  options: SystemSshWriteBufferOptions,
-  offset: number
-): Promise<void> {
-  throwIfAborted(options.signal)
-  const channel = spawnSystemSshCommand(target, makeWindowsWriteFileCommand(remotePath, options), {
-    wrapCommand: false,
-    ...getSystemSshBuildArgsFromOperationOptions(options)
-  })
-  const closePromise = awaitWithSystemSshAbort(
-    options.signal,
-    () => channel.close(),
-    waitForChannelClose(
-      channel,
-      `write ${remotePath} at offset ${offset}`,
-      WINDOWS_STDIN_WRITE_TIMEOUT_MS
-    )
-  )
-  if (!options.signal?.aborted) {
-    channel.stdin.end(chunk)
-  }
-  await closePromise
-}
-
-async function publishWindowsStagedWrite(
-  target: SshTarget,
-  stagingPath: string,
-  remotePath: string,
-  options: SystemSshWriteBufferOptions
-): Promise<void> {
-  throwIfAborted(options.signal)
-  const channel = spawnSystemSshCommand(
-    target,
-    makeWindowsPublishStagedFileCommand(stagingPath, remotePath, options.exclusive === true),
-    { wrapCommand: false, ...getSystemSshBuildArgsFromOperationOptions(options) }
-  )
-  const closePromise = awaitWithSystemSshAbort(
-    options.signal,
-    () => channel.close(),
-    waitForChannelClose(channel, `publish ${remotePath}`, WINDOWS_STDIN_WRITE_TIMEOUT_MS)
-  )
-  if (!options.signal?.aborted) {
-    channel.stdin.end()
-  }
-  await closePromise
-}
-
-function makeWindowsWriteFileCommand(
-  remotePath: string,
-  options?: { append?: boolean; exclusive?: boolean }
-): string {
-  const fileMode = options?.append ? 'Append' : options?.exclusive ? 'CreateNew' : 'Create'
-  return powerShellCommand(
-    [
-      '$ErrorActionPreference = "Stop"',
-      `$path = ${powerShellLiteral(remotePath)}`,
-      '$parent = [System.IO.Path]::GetDirectoryName($path)',
-      'if ($parent) { $null = [System.IO.Directory]::CreateDirectory($parent) }',
-      '$inputStream = [Console]::OpenStandardInput()',
-      `$outputStream = [System.IO.File]::Open($path, [System.IO.FileMode]::${fileMode}, [System.IO.FileAccess]::Write, [System.IO.FileShare]::None)`,
-      'try { $inputStream.CopyTo($outputStream) } finally { $outputStream.Dispose() }'
-    ].join('; ')
-  )
-}
-
-// `File::Move` throws when the destination exists, which is exactly the exclusive contract; the
-// non-exclusive caller asked to replace, so it deletes first (a no-op on an absent path).
-function makeWindowsPublishStagedFileCommand(
-  stagingPath: string,
-  remotePath: string,
-  exclusive: boolean
-): string {
-  return powerShellCommand(
-    [
-      '$ErrorActionPreference = "Stop"',
-      `$staging = ${powerShellLiteral(stagingPath)}`,
-      `$path = ${powerShellLiteral(remotePath)}`,
-      ...(exclusive ? [] : ['[System.IO.File]::Delete($path)']),
-      '[System.IO.File]::Move($staging, $path)'
-    ].join('; ')
-  )
-}
-
 function makePosixWriteFileCommand(
   remotePath: string,
   options?: { append?: boolean; exclusive?: boolean }
diff --git a/src/main/ssh/system-ssh-file-transfer.ts b/src/main/ssh/system-ssh-file-transfer.ts
index f728c0eab02..052ac5a79a3 100644
--- a/src/main/ssh/system-ssh-file-transfer.ts
+++ b/src/main/ssh/system-ssh-file-transfer.ts
@@ -27,6 +27,8 @@ import {
   WINDOWS_STDIN_WRITE_TIMEOUT_MS,
   writeBufferViaSystemSsh
 } from './system-ssh-file-binary-transfer'
+import { isSftpUnavailableError, makeDirectoriesViaSftp } from './system-ssh-sftp-transfer'
+import { getWindowsRemoteWriteCapabilities } from './system-ssh-windows-write-capabilities'
 
 type SystemSshOperationOptions = SystemSshBuildArgsOptions & {
   signal?: AbortSignal
@@ -161,9 +163,14 @@ async function collectWindowsUploadPlan(
   return plan
 }
 
-// Why the JSON envelope survives here: a path list is metadata, so this payload stays in the
-// hundreds of bytes even for a deep tree. Batched anyway, so a pathological tree cannot walk back
-// into the same stdin size that wedges PowerShell.
+/**
+ * Creates the upload's directories, preferring sftp's own `mkdir`.
+ *
+ * The PowerShell fallback keeps the JSON envelope, batched under one stdin's worth: a path list is
+ * metadata, so it stays in the hundreds of bytes even for a deep tree. It is still a redirected
+ * stdin read though, so on Windows PowerShell 5.1 it carries the same defect as any other — which
+ * is why sftp is tried first even for a payload this small.
+ */
 async function createWindowsUploadDirectories(
   target: SshTarget,
   directories: readonly string[],
@@ -175,23 +182,17 @@ async function createWindowsUploadDirectories(
     if (batch.length === 0) {
       return
     }
+    const pending = batch
     const payload = JSON.stringify(batch)
     batch = []
     batchBytes = 0
     throwIfAborted(options.signal)
-    const channel = spawnSystemSshCommand(target, makeWindowsCreateDirectoriesCommand(), {
-      wrapCommand: false,
-      ...getSystemSshBuildArgsFromOperationOptions(options)
-    })
-    const closePromise = awaitWithSystemSshAbort(
-      options.signal,
-      () => channel.close(),
-      waitForChannelClose(channel, 'windows relay upload mkdir', WINDOWS_STDIN_WRITE_TIMEOUT_MS)
+    await getWindowsRemoteWriteCapabilities(target).runWithFallback(
+      'sftp-subsystem',
+      () => makeDirectoriesViaSftp(target, pending, options),
+      () => createWindowsUploadDirectoriesViaPowerShell(target, payload, options),
+      isSftpUnavailableError
     )
-    if (!options.signal?.aborted) {
-      channel.stdin.end(payload)
-    }
-    await closePromise
   }
   for (const directory of directories) {
     const entryBytes = Buffer.byteLength(directory) + 4
@@ -204,12 +205,33 @@ async function createWindowsUploadDirectories(
   await flush()
 }
 
+async function createWindowsUploadDirectoriesViaPowerShell(
+  target: SshTarget,
+  payload: string,
+  options: SystemSshOperationOptions
+): Promise<void> {
+  const channel = spawnSystemSshCommand(target, makeWindowsCreateDirectoriesCommand(), {
+    wrapCommand: false,
+    ...getSystemSshBuildArgsFromOperationOptions(options)
+  })
+  const closePromise = awaitWithSystemSshAbort(
+    options.signal,
+    () => channel.close(),
+    waitForChannelClose(channel, 'windows relay upload mkdir', WINDOWS_STDIN_WRITE_TIMEOUT_MS)
+  )
+  if (!options.signal?.aborted) {
+    channel.stdin.end(payload)
+  }
+  await closePromise
+}
+
 function makeWindowsCreateDirectoriesCommand(): string {
   return powerShellCommand(
     [
       '$ErrorActionPreference = "Stop"',
-      // The reporter measured this reader surviving 50KB where `[Console]::In` wedged at the same
-      // size (#16432); the batch above stays under that.
+      // Reached only where the host has no sftp subsystem. Windows PowerShell 5.1 can lose a
+      // redirected stdin for good when a read finds it empty (#16432); a batch this small usually
+      // arrives in one piece, and "usually" is exactly why sftp is preferred.
       '$reader = New-Object System.IO.StreamReader([Console]::OpenStandardInput())',
       'try { $json = $reader.ReadToEnd() } finally { $reader.Dispose() }',
       'if ([string]::IsNullOrWhiteSpace($json)) { return }',
diff --git a/src/main/ssh/system-ssh-sftp-args.test.ts b/src/main/ssh/system-ssh-sftp-args.test.ts
new file mode 100644
index 00000000000..94db6ebbea5
--- /dev/null
+++ b/src/main/ssh/system-ssh-sftp-args.test.ts
@@ -0,0 +1,100 @@
+/**
+ * `buildSshArgs` is shared with the sftp client, and three of its flags mean something else there.
+ * Every case below is a silent wrong-target rather than an error if the translation is skipped,
+ * which is why the fallback is "refuse and use another transport", never "pass it through".
+ */
+import { describe, expect, it } from 'vitest'
+import {
+  SftpArgTranslationError,
+  translateSshArgsToSftpArgs,
+  withSftpKeepalive
+} from './system-ssh-sftp-args'
+
+describe('translateSshArgsToSftpArgs', () => {
+  it('sends the port as an option, since sftp -p preserves mtimes instead', () => {
+    const args = translateSshArgsToSftpArgs(['-p', '2222', '--', 'dev@win.example'])
+
+    expect(args).toEqual(['-o', 'Port=2222', '--', 'dev@win.example'])
+  })
+
+  it('spells ControlPath=none out, since sftp -S names a program to run', () => {
+    // `sftp -S none` would try to exec a binary called `none`.
+    const args = translateSshArgsToSftpArgs(['-S', 'none', '--', 'dev@win.example'])
+
+    expect(args).toEqual(['-o', 'ControlPath=none', '--', 'dev@win.example'])
+  })
+
+  it('refuses any other -S, which would hand sftp an ssh binary Orca did not choose', () => {
+    expect(() => translateSshArgsToSftpArgs(['-S', '/tmp/ctl.sock'])).toThrow(
+      SftpArgTranslationError
+    )
+  })
+
+  it('drops -T, which sftp does not have', () => {
+    expect(translateSshArgsToSftpArgs(['-T', '--', 'host'])).toEqual(['--', 'host'])
+  })
+
+  it('passes through the flags both clients spell the same way', () => {
+    const args = translateSshArgsToSftpArgs([
+      '-F',
+      '/tmp/config',
+      '-o',
+      'BatchMode=yes',
+      '-i',
+      '/tmp/key',
+      '-J',
+      'jump.example',
+      '--',
+      'dev@win.example'
+    ])
+
+    expect(args).toEqual([
+      '-F',
+      '/tmp/config',
+      '-o',
+      'BatchMode=yes',
+      '-i',
+      '/tmp/key',
+      '-J',
+      'jump.example',
+      '--',
+      'dev@win.example'
+    ])
+  })
+
+  it('takes everything after -- as the destination without reinterpreting it', () => {
+    // A host literally named `-p` is not a flag once `--` has been seen.
+    expect(translateSshArgsToSftpArgs(['--', '-p'])).toEqual(['--', '-p'])
+  })
+
+  it('refuses an unknown flag rather than guessing what sftp would do with it', () => {
+    // The point of the throw: a flag added to buildSshArgs later must degrade to another
+    // transport, not reach sftp carrying a different meaning.
+    expect(() => translateSshArgsToSftpArgs(['-A', '--', 'host'])).toThrow(SftpArgTranslationError)
+  })
+
+  it('refuses a value flag with no value', () => {
+    expect(() => translateSshArgsToSftpArgs(['-o'])).toThrow(SftpArgTranslationError)
+  })
+})
+
+describe('withSftpKeepalive', () => {
+  it('asks OpenSSH to notice a dead peer, since the transfer itself has no wall-clock bound', () => {
+    expect(withSftpKeepalive(['--', 'host'])).toEqual([
+      '-o',
+      'ServerAliveInterval=15',
+      '-o',
+      'ServerAliveCountMax=3',
+      '--',
+      'host'
+    ])
+  })
+
+  it('leaves a caller-stated keepalive policy alone', () => {
+    const args = withSftpKeepalive(['-o', 'ServerAliveInterval=60', '--', 'host'])
+
+    expect(args.filter((arg) => arg.startsWith('ServerAliveInterval'))).toEqual([
+      'ServerAliveInterval=60'
+    ])
+  })
+})
diff --git a/src/main/ssh/system-ssh-sftp-args.ts b/src/main/ssh/system-ssh-sftp-args.ts
new file mode 100644
index 00000000000..55cb01184dc
--- /dev/null
+++ b/src/main/ssh/system-ssh-sftp-args.ts
@@ -0,0 +1,84 @@
+/**
+ * Rewrites `buildSshArgs` output for the sftp(1) client.
+ *
+ * Three flags ssh and sftp share spell different things: sftp's `-p` is "preserve mtime", its `-S`
+ * names the ssh binary to run, and it has no `-T` at all. Passing ssh's list through unchanged
+ * would silently connect to the wrong port and try to exec a program called `none`.
+ *
+ * Anything this table does not recognize throws. A flag added to `buildSshArgs` later must degrade
+ * to the non-sftp transfer path, never reach sftp carrying a different meaning.
+ */
+
+/** `buildSshArgs` emitted a flag with no sftp equivalent; the caller should use another transport. */
+export class SftpArgTranslationError extends Error {
+  constructor(flag: string) {
+    super(`No sftp equivalent for system ssh argument ${JSON.stringify(flag)}`)
+    this.name = 'SftpArgTranslationError'
+  }
+}
+
+/** Flags whose spelling and meaning are identical in both clients. */
+const PASSTHROUGH_VALUE_FLAGS = new Set(['-F', '-o', '-i', '-J'])
+
+export function translateSshArgsToSftpArgs(sshArgs: readonly string[]): string[] {
+  const sftpArgs: string[] = []
+  let index = 0
+  while (index < sshArgs.length) {
+    const flag = sshArgs[index]!
+    if (flag === '--') {
+      // Everything after `--` is the destination, which both clients spell the same way.
+      sftpArgs.push(...sshArgs.slice(index))
+      return sftpArgs
+    }
+    const value = sshArgs[index + 1]
+    if (PASSTHROUGH_VALUE_FLAGS.has(flag)) {
+      if (value === undefined) {
+        throw new SftpArgTranslationError(flag)
+      }
+      sftpArgs.push(flag, value)
+      index += 2
+      continue
+    }
+    if (flag === '-T') {
+      // sftp never allocates a tty, so ssh's "no tty" request has nothing to translate to.
+      index += 1
+      continue
+    }
+    if (flag === '-p') {
+      if (value === undefined) {
+        throw new SftpArgTranslationError(flag)
+      }
+      sftpArgs.push('-o', `Port=${value}`)
+      index += 2
+      continue
+    }
+    if (flag === '-S') {
+      // ssh's `-S none` is ControlPath=none; sftp's `-S` would run a binary called `none`.
+      if (value !== 'none') {
+        throw new SftpArgTranslationError(flag)
+      }
+      sftpArgs.push('-o', 'ControlPath=none')
+      index += 2
+      continue
+    }
+    throw new SftpArgTranslationError(flag)
+  }
+  return sftpArgs
+}
+
+/**
+ * A transfer that stalls mid-stream has no per-write bound to catch it, so ask OpenSSH to notice a
+ * dead peer itself. Only added when the caller has not already stated a keepalive policy.
+ */
+export function withSftpKeepalive(sftpArgs: readonly string[]): string[] {
+  const hasOption = (name: string): boolean =>
+    sftpArgs.some((arg, position) => sftpArgs[position - 1] === '-o' && arg.startsWith(`${name}=`))
+  const keepalive: string[] = []
+  if (!hasOption('ServerAliveInterval')) {
+    keepalive.push('-o', 'ServerAliveInterval=15')
+  }
+  if (!hasOption('ServerAliveCountMax')) {
+    keepalive.push('-o', 'ServerAliveCountMax=3')
+  }
+  return [...keepalive, ...sftpArgs]
+}
diff --git a/src/main/ssh/system-ssh-sftp-path.test.ts b/src/main/ssh/system-ssh-sftp-path.test.ts
new file mode 100644
index 00000000000..e196c2e3e8f
--- /dev/null
+++ b/src/main/ssh/system-ssh-sftp-path.test.ts
@@ -0,0 +1,59 @@
+/**
+ * Both functions here guard against the same measured failure: sftp's batch lexer treats `\` as an
+ * escape, so a Windows path handed over raw is silently mis-targeted *and the client still exits
+ * 0*. On Windows 11 / OpenSSH 10.0p2, `put src C:\Users\neil\qt\a.bin` created a file literally
+ * named `C` in the start directory and reported success.
+ */
+import { describe, expect, it } from 'vitest'
+import {
+  quoteSftpBatchArgument,
+  toSftpRemotePath,
+  UnsupportedSftpPathError
+} from './system-ssh-sftp-path'
+
+describe('toSftpRemotePath', () => {
+  it('roots a drive path under /, which is the namespace the Windows sftp-server exposes', () => {
+    // `pwd` in that session reports `/C:/Users/dev`.
+    expect(toSftpRemotePath('C:/Users/dev/f.bin')).toBe('/C:/Users/dev/f.bin')
+  })
+
+  it('accepts a path already in that namespace unchanged', () => {
+    expect(toSftpRemotePath('/C:/Users/dev/f.bin')).toBe('/C:/Users/dev/f.bin')
+  })
+
+  it('converts the separators Orca stores paths with', () => {
+    expect(toSftpRemotePath('C:\\Users\\dev\\f.bin')).toBe('/C:/Users/dev/f.bin')
+  })
+
+  it('declines a UNC path rather than guessing where it lands', () => {
+    // A guess here writes real bytes to the wrong place; declining falls back to another transport.
+    expect(() => toSftpRemotePath('//server/share/f.bin')).toThrow(UnsupportedSftpPathError)
+  })
+
+  it('declines a relative path, which would resolve against the session start directory', () => {
+    expect(() => toSftpRemotePath('Users/dev/f.bin')).toThrow(UnsupportedSftpPathError)
+  })
+})
+
+describe('quoteSftpBatchArgument', () => {
+  it('escapes the backslashes in a Windows client local path', () => {
+    // Unescaped, sftp reads this as C:srcf.bin and fails to find the source.
+    expect(quoteSftpBatchArgument('C:\\src\\f.bin')).toBe('"C:\\\\src\\\\f.bin"')
+  })
+
+  it('keeps a path with spaces as one argument', () => {
+    expect(quoteSftpBatchArgument('/tmp/two words.bin')).toBe('"/tmp/two words.bin"')
+  })
+
+  it('escapes an embedded quote, which would otherwise end the argument early', () => {
+    expect(quoteSftpBatchArgument('/tmp/dq".bin')).toBe('"/tmp/dq\\".bin"')
+  })
+
+  it('refuses a line break, which would split one batch command into two', () => {
+    expect(() => quoteSftpBatchArgument('/tmp/a\nrm -rf b')).toThrow(UnsupportedSftpPathError)
+  })
+
+  it('refuses a NUL, which truncates the argument', () => {
+    expect(() => quoteSftpBatchArgument('/tmp/a\0b')).toThrow(UnsupportedSftpPathError)
+  })
+})
diff --git a/src/main/ssh/system-ssh-sftp-path.ts b/src/main/ssh/system-ssh-sftp-path.ts
new file mode 100644
index 00000000000..2b5bfe53f02
--- /dev/null
+++ b/src/main/ssh/system-ssh-sftp-path.ts
@@ -0,0 +1,46 @@
+import { normalizeWindowsRemotePath } from './ssh-remote-platform'
+
+/**
+ * A path this transfer cannot express to sftp. Callers treat it as "use another transport", never
+ * as a transfer failure.
+ */
+export class UnsupportedSftpPathError extends Error {
+  constructor(path: string) {
+    super(`Path cannot be addressed over sftp: ${JSON.stringify(path)}`)
+    this.name = 'UnsupportedSftpPathError'
+  }
+}
+
+/**
+ * Converts a Windows remote path to the namespace OpenSSH's Windows sftp-server exposes, which
+ * roots every drive under `/`: `C:/Users/dev/f` is `/C:/Users/dev/f`, and `pwd` there reports
+ * `/C:/Users/dev`.
+ */
+export function toSftpRemotePath(remotePath: string): string {
+  const normalized = normalizeWindowsRemotePath(remotePath)
+  if (/^\/[a-zA-Z]:\//.test(normalized)) {
+    return normalized
+  }
+  if (/^[a-zA-Z]:\//.test(normalized)) {
+    return `/${normalized}`
+  }
+  // UNC (`//server/share`) and relative paths have no settled mapping in this namespace, and a
+  // guess here writes real bytes to the wrong place. Decline instead.
+  throw new UnsupportedSftpPathError(remotePath)
+}
+
+/**
+ * Quotes one argument of an sftp batch line.
+ *
+ * Escaping is load-bearing, not cosmetic: sftp's batch lexer treats `\` as an escape even inside
+ * double quotes, so an unescaped Windows local path `C:\src\f.bin` is read as `C:srcf.bin`, and an
+ * unescaped destination `C:\Users\dev\f.bin` writes a file literally named `C` in the start
+ * directory — while sftp still exits 0. Both measured on Windows 11 / OpenSSH 10.0p2.
+ */
+export function quoteSftpBatchArgument(value: string): string {
+  if (/[\n\r\0]/.test(value)) {
+    // A line break would split one batch command into two; NUL truncates the argument.
+    throw new UnsupportedSftpPathError(value)
+  }
+  return `"${value.replace(/([\\"])/g, '\\$1')}"`
+}
diff --git a/src/main/ssh/system-ssh-sftp-transfer.ts b/src/main/ssh/system-ssh-sftp-transfer.ts
new file mode 100644
index 00000000000..2e5c6f6dcb1
--- /dev/null
+++ b/src/main/ssh/system-ssh-sftp-transfer.ts
@@ -0,0 +1,176 @@
+import { accessSync, constants, existsSync, statSync } from 'node:fs'
+import { posix, win32 } from 'node:path'
+import type { SshTarget } from '../../shared/ssh-types'
+import { buildSshArgs, type SystemSshBuildArgsOptions } from './system-ssh-args'
+import { findSystemSsh } from './system-ssh-binary'
+import {
+  SftpArgTranslationError,
+  translateSshArgsToSftpArgs,
+  withSftpKeepalive
+} from './system-ssh-sftp-args'
+import {
+  quoteSftpBatchArgument,
+  toSftpRemotePath,
+  UnsupportedSftpPathError
+} from './system-ssh-sftp-path'
+import { throwIfAborted } from './system-ssh-operation-lifecycle'
+import { runProcess } from '../../shared/child-process/run-process'
+
+/** The host answered, but not with an sftp subsystem. The caller must fall back, not fail. */
+export class SftpSubsystemUnavailableError extends Error {
+  constructor(detail: string) {
+    super(`Remote host has no usable sftp subsystem: ${detail}`)
+    this.name = 'SftpSubsystemUnavailableError'
+  }
+}
+
+/**
+ * True for the errors that mean "this host cannot do sftp", as opposed to "this transfer failed".
+ * Deliberately narrow: a permission denial or a missing directory is a real failure that must
+ * surface, not a reason to retry the whole upload down a slower path.
+ */
+export function isSftpUnavailableError(error: unknown): boolean {
+  return (
+    error instanceof SftpSubsystemUnavailableError ||
+    error instanceof SftpArgTranslationError ||
+    error instanceof UnsupportedSftpPathError
+  )
+}
+
+function systemSftpCandidates(sshPath: string | null, platform: NodeJS.Platform): string[] {
+  const pathApi = platform === 'win32' ? win32 : posix
+  const executable = platform === 'win32' ? 'sftp.exe' : 'sftp'
+  const candidates: string[] = []
+  // Why the ssh binary's own directory first: a host with two OpenSSH installs must pair the sftp
+  // client with the ssh that `buildSshArgs` was built for, not whichever one PATH happens to reach.
+  if (sshPath) {
+    candidates.push(pathApi.join(pathApi.dirname(sshPath), executable))
+  }
+  if (platform === 'win32') {
+    const systemRoot = process.env.SystemRoot || process.env.WINDIR
+    if (systemRoot) {
+      candidates.push(win32.join(systemRoot, 'System32', 'OpenSSH', executable))
+    }
+  } else {
+    candidates.push('/usr/bin/sftp', '/usr/local/bin/sftp', '/opt/homebrew/bin/sftp')
+  }
+  return candidates
+}
+
+/** Locate the sftp client paired with the system ssh binary. Returns null when there is none. */
+export function findSystemSftp(): string | null {
+  if (process.env.ORCA_SYSTEM_SFTP_PATH) {
+    return process.env.ORCA_SYSTEM_SFTP_PATH
+  }
+  const sshPath = findSystemSsh()
+  for (const candidate of systemSftpCandidates(sshPath, process.platform)) {
+    try {
+      if (!statSync(candidate).isFile()) {
+        continue
+      }
+      if (process.platform !== 'win32') {
+        accessSync(candidate, constants.X_OK)
+      }
+      return candidate
+    } catch {
+      continue
+    }
+  }
+  return findSftpOnPath()
+}
+
+function findSftpOnPath(): string | null {
+  const pathValue = process.env.PATH
+  if (!pathValue) {
+    return null
+  }
+  const pathApi = process.platform === 'win32' ? win32 : posix
+  const executable = process.platform === 'win32' ? 'sftp.exe' : 'sftp'
+  for (const entry of pathValue.split(pathApi.delimiter)) {
+    const directory = entry.trim().replace(/^"|"$/g, '')
+    if (!directory) {
+      continue
+    }
+    const candidate = pathApi.join(directory, executable)
+    if (existsSync(candidate)) {
+      return candidate
+    }
+  }
+  return null
+}
+
+/**
+ * OpenSSH prints this when the server refuses the subsystem — a host with `Subsystem sftp`
+ * commented out, or an internal-sftp block that does not apply to this user.
+ */
+const SUBSYSTEM_REFUSED_PATTERN = /subsystem request failed|no such file or directory.*sftp-server/i
+
+export type SftpBatchOptions = SystemSshBuildArgsOptions & { signal?: AbortSignal }
+
+/**
+ * Runs one sftp batch script.
+ *
+ * The script goes to the *local* sftp client's stdin, which is the point: no remote process ever
+ * reads a redirected stdin, so none of this rides the Windows PowerShell stdin defect.
+ */
+export async function runSftpBatch(
+  target: SshTarget,
+  commands: readonly string[],
+  options?: SftpBatchOptions
+): Promise<void> {
+  throwIfAborted(options?.signal)
+  const sftpPath = findSystemSftp()
+  if (!sftpPath) {
+    throw new SftpSubsystemUnavailableError('no sftp client binary found alongside ssh')
+  }
+  const args = withSftpKeepalive(translateSshArgsToSftpArgs(buildSshArgs(target, options)))
+  let result
+  try {
+    result = await runProcess({
+      program: sftpPath,
+      args: ['-b', '-', ...args],
+      // `-b -` takes the script on stdin, and that stdin is the *local* client's — no remote
+      // process reads a pipe anywhere in this transfer, which is the whole point of preferring it.
+      input: `${commands.join('\n')}\n`,
+      // Why no timeout: a large upload is legitimately slow, and a wall-clock cap would fail a
+      // healthy transfer on a slow link. A dead peer is caught by the ServerAlive options instead.
+      timeoutMs: null,
+      signal: options?.signal
+    })
+  } catch (error) {
+    // A client that will not start is "this host cannot do sftp" from the caller's side, not a
+    // transfer failure: the payload never left. Falling back is the only useful answer.
+    throw new SftpSubsystemUnavailableError(
+      `sftp client at ${sftpPath} could not be started: ${error instanceof Error ? error.message : String(error)}`
+    )
+  }
+  if (result.code === 0) {
+    return
+  }
+  throwIfAborted(options?.signal)
+  const detail = result.stderr.trim()
+  if (SUBSYSTEM_REFUSED_PATTERN.test(detail)) {
+    throw new SftpSubsystemUnavailableError(detail)
+  }
+  throw new Error(`sftp batch failed (exit ${result.code}): ${detail}`)
+}
+
+/**
+ * Creates remote directories, parents first.
+ *
+ * `-mkdir` keeps sftp going when a directory is already there; batch mode otherwise aborts the
+ * whole script on the first non-zero status, which for an idempotent tree walk is not a failure.
+ */
+export function makeDirectoriesViaSftp(
+  target: SshTarget,
+  remoteDirectories: readonly string[],
+  options?: SftpBatchOptions
+): Promise<void> {
+  const commands = remoteDirectories.map(
+    (directory) => `-mkdir ${quoteSftpBatchArgument(toSftpRemotePath(directory))}`
+  )
+  if (commands.length === 0) {
+    return Promise.resolve()
+  }
+  return runSftpBatch(target, commands, options)
+}
diff --git a/src/main/ssh/system-ssh-windows-file-write.ts b/src/main/ssh/system-ssh-windows-file-write.ts
new file mode 100644
index 00000000000..7b2cb1f015e
--- /dev/null
+++ b/src/main/ssh/system-ssh-windows-file-write.ts
@@ -0,0 +1,121 @@
+import { randomBytes } from 'node:crypto'
+import { powerShellCommand, powerShellLiteral } from './ssh-remote-powershell'
+import { normalizeWindowsRemotePath } from './ssh-remote-platform'
+
+/**
+ * Suffix marking the path a Windows write lands on before it is published by rename.
+ *
+ * The random tail is the fix for a measured harm, not decoration. A write that loses contact with
+ * the host leaves a remote process that may still hold the staging file open exclusively, and
+ * `docs/reference/ssh-execution-boundary.md` is explicit that losing contact is not evidence that
+ * process died — so the retry must not reuse the name it may still own. A fresh name per attempt
+ * means a retry never meets its predecessor's lock; the abandoned file is cleaned up best-effort
+ * and never treated as proof of anything.
+ */
+export const WINDOWS_STAGED_WRITE_SUFFIX = '.orca-partial'
+
+export function makeWindowsStagingPath(remotePath: string): string {
+  return `${remotePath}${WINDOWS_STAGED_WRITE_SUFFIX}-${randomBytes(6).toString('hex')}`
+}
+
+export type WindowsPublishMode = 'create' | 'exclusive' | 'append'
+
+/**
+ * Publishes a staged upload onto its real name.
+ *
+ * Every branch reads the staged *file*, never a redirected stdin, which is what makes this safe on
+ * a host whose Windows PowerShell 5.1 cannot drain a piped stdin. `File::Move` throws when the
+ * destination exists, which is exactly the exclusive contract; the replacing caller deletes first
+ * (a no-op on an absent path).
+ */
+export function makeWindowsPublishStagedFileCommand(
+  stagingPath: string,
+  remotePath: string,
+  mode: WindowsPublishMode
+): string {
+  const preamble = [
+    '$ErrorActionPreference = "Stop"',
+    `$staging = ${powerShellLiteral(stagingPath)}`,
+    `$path = ${powerShellLiteral(remotePath)}`,
+    '$parent = [System.IO.Path]::GetDirectoryName($path)',
+    'if ($parent) { $null = [System.IO.Directory]::CreateDirectory($parent) }'
+  ]
+  if (mode === 'append') {
+    return powerShellCommand(
+      [
+        ...preamble,
+        '$in = [System.IO.File]::OpenRead($staging)',
+        '$out = [System.IO.File]::Open($path, [System.IO.FileMode]::Append, [System.IO.FileAccess]::Write, [System.IO.FileShare]::None)',
+        'try { $in.CopyTo($out) } finally { $out.Dispose(); $in.Dispose() }',
+        '[System.IO.File]::Delete($staging)'
+      ].join('; ')
+    )
+  }
+  return powerShellCommand(
+    [
+      ...preamble,
+      ...(mode === 'exclusive' ? [] : ['[System.IO.File]::Delete($path)']),
+      '[System.IO.File]::Move($staging, $path)'
+    ].join('; ')
+  )
+}
+
+/** Best-effort removal of a staged file whose write was abandoned. Never asserts the writer died. */
+export function makeWindowsDiscardStagedFileCommand(stagingPath: string): string {
+  return powerShellCommand(
+    [
+      // Deliberately not `Stop`: the previous writer may still hold this file, and that is a
+      // possibility to tolerate, not an error to report. The unique staging name means a leftover
+      // blocks nothing; sweeping it is housekeeping.
+      '$ErrorActionPreference = "SilentlyContinue"',
+      `$staging = ${powerShellLiteral(stagingPath)}`,
+      '[System.IO.File]::Delete($staging)'
+    ].join('; ')
+  )
+}
+
+/**
+ * The ancestor directories of a Windows remote path, drive root first.
+ *
+ * sftp's `mkdir` creates one level, so a batch has to name each level itself. The drive root is
+ * excluded: `-mkdir "/C:/"` is not a directory anyone creates.
+ */
+export function windowsRemoteAncestorDirectories(remotePath: string): string[] {
+  const normalized = normalizeWindowsRemotePath(remotePath)
+  const segments = normalized.split('/')
+  segments.pop()
+  const ancestors: string[] = []
+  // Start past the drive (`C:`) or the UNC host, which are never created.
+  for (let depth = 2; depth <= segments.length; depth += 1) {
+    const directory = segments.slice(0, depth).join('/')
+    if (directory) {
+      ancestors.push(directory)
+    }
+  }
+  return ancestors
+}
+
+/**
+ * `[Console]::OpenStandardInput()` into a `FileStream`, used only by the two stdin fallbacks.
+ *
+ * On Windows PowerShell 5.1 this is the defective read; see the strategy comment in
+ * `system-ssh-file-binary-transfer.ts`. It is correct under PowerShell 7.
+ */
+export function makeWindowsWriteFileCommand(
+  remotePath: string,
+  options?: { append?: boolean; exclusive?: boolean; executable?: 'powershell.exe' | 'pwsh.exe' }
+): string {
+  const fileMode = options?.append ? 'Append' : options?.exclusive ? 'CreateNew' : 'Create'
+  return powerShellCommand(
+    [
+      '$ErrorActionPreference = "Stop"',
+      `$path = ${powerShellLiteral(remotePath)}`,
+      '$parent = [System.IO.Path]::GetDirectoryName($path)',
+      'if ($parent) { $null = [System.IO.Directory]::CreateDirectory($parent) }',
+      '$inputStream = [Console]::OpenStandardInput()',
+      `$outputStream = [System.IO.File]::Open($path, [System.IO.FileMode]::${fileMode}, [System.IO.FileAccess]::Write, [System.IO.FileShare]::None)`,
+      'try { $inputStream.CopyTo($outputStream) } finally { $outputStream.Dispose() }'
+    ].join('; '),
+    options?.executable ?? 'powershell.exe'
+  )
+}
diff --git a/src/main/ssh/system-ssh-windows-upload.test.ts b/src/main/ssh/system-ssh-windows-upload.test.ts
index 207c3e2df8e..82fed2a9b57 100644
--- a/src/main/ssh/system-ssh-windows-upload.test.ts
+++ b/src/main/ssh/system-ssh-windows-upload.test.ts
@@ -1,29 +1,40 @@
 /**
- * #16432: the Windows relay upload pushed the whole bundle into one PowerShell stdin, which
- * Windows PowerShell 5.1 cannot drain over a non-pty ssh exec — the remote blocks forever, and
- * `waitForChannelClose()` had no timeout, so the UI sat at "Connecting…" with no error. Covered
- * here: no write exceeds one stdin's worth on any Windows path (bundle upload *and* single-file
- * upload, which is the one that carries large files), a partial write never lands under the real
- * name, and a remote that never closes fails instead of hanging.
+ * #16432. The original fix chunked the payload because the constraint was believed to be a ~50KB
+ * cmd.exe stdin ceiling. Re-measured on Windows 11 26200.9168 / OpenSSH_for_Windows_10.0p2, it is
+ * not a size limit and not cmd.exe's: a read on Windows PowerShell 5.1's redirected-stdin handle
+ * over a non-pty ssh exec can die permanently when it finds the stream momentarily empty, taking
+ * both the remaining data and the EOF with it. It is probabilistic per such read — identical 2MB
+ * payloads died at 167936, 270336 and 372736 — so a 32KB chunk still failed 15 times in 120 under
+ * load, while `findstr` took 2,016,000 bytes through one exec on the same host.
+ *
+ * So the covering property is no longer "every write is small". It is "the bytes do not cross a
+ * remote process's stdin at all": sftp first, PowerShell 7 next, and Windows PowerShell 5.1 last,
+ * bounded and loud. The staging-and-rename discipline is kept on every path, with a unique staging
+ * name per attempt so a retry never meets a predecessor's lock.
  */
 import { EventEmitter } from 'node:events'
 import { mkdirSync, mkdtempSync, writeFileSync } from 'node:fs'
-import { rm } from 'node:fs/promises'
+import { readFile, rm } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { PassThrough, Writable } from 'node:stream'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import type * as SystemSshOperationLifecycle from './system-ssh-operation-lifecycle'
 
-const { spawnSystemSshCommandMock, waitForChannelCloseSpy } = vi.hoisted(() => ({
+const { spawnSystemSshCommandMock, waitForChannelCloseSpy, runProcessMock } = vi.hoisted(() => ({
   spawnSystemSshCommandMock: vi.fn(),
-  waitForChannelCloseSpy: vi.fn()
+  waitForChannelCloseSpy: vi.fn(),
+  runProcessMock: vi.fn()
 }))
 
 vi.mock('./system-ssh-command', () => ({
   spawnSystemSshCommand: spawnSystemSshCommandMock
 }))
 
+vi.mock('../../shared/child-process/run-process', () => ({
+  runProcess: runProcessMock
+}))
+
 // Delegates to the real implementation; the spy only records whether each wait was given a bound.
 vi.mock('./system-ssh-operation-lifecycle', async (importActual) => {
   const actual = (await importActual()) as typeof SystemSshOperationLifecycle
@@ -41,6 +52,11 @@ import {
 } from './system-ssh-file-binary-transfer'
 import { waitForChannelClose } from './system-ssh-operation-lifecycle'
 import { getRemoteHostPlatform } from './ssh-remote-platform'
+import {
+  clearWindowsRemoteWriteCapabilitiesForTests,
+  getWindowsRemoteWriteCapabilities
+} from './system-ssh-windows-write-capabilities'
+import { explainWindowsPowerShellStdinFailure } from './system-ssh-windows-write-strategy'
 import type { SshTarget } from '../../shared/ssh-types'
 
 type FakeChannel = EventEmitter & {
@@ -50,7 +66,12 @@ type FakeChannel = EventEmitter & {
   written: Buffer
 }
 
-const target = { id: 'win-1', host: 'win.example', username: 'dev' } as unknown as SshTarget
+const target = {
+  id: 'win-1',
+  host: 'win.example',
+  username: 'dev',
+  port: 22
+} as unknown as SshTarget
 const hostPlatform = getRemoteHostPlatform('win32-x64')
 const remoteRoot = 'C:/Users/dev/.orca-remote'
 
@@ -78,96 +99,212 @@ function createFakeChannel(onEnd: (channel: FakeChannel) => void): FakeChannel {
   return channel
 }
 
-type RecordedCommand = { script: string; stdin: Buffer }
+type RecordedCommand = { script: string; executable: string; stdin: Buffer }
+type RecordedSftpBatch = { args: string[]; script: string }
 
-describe('Windows upload stdin framing', () => {
-  let localDir: string
-  const commands: RecordedCommand[] = []
-  /** Index of the spawn that should report a non-zero exit, to model a chunk failing mid-file. */
-  let failAtSpawn = -1
+const sftpBatches: RecordedSftpBatch[] = []
+const commands: RecordedCommand[] = []
+/** Index of the exec that should report a non-zero exit, to model a chunk failing mid-file. */
+let failAtSpawn = -1
+let localDir: string
 
-  const fileWrites = (): RecordedCommand[] =>
-    commands.filter((command) => command.script.includes('FileMode]::'))
-  const writtenPath = (command: RecordedCommand): string =>
-    /\$path = '((?:[^']|'')*)'/.exec(command.script)?.[1].replace(/''/g, "'") ?? ''
-  const fileMode = (command: RecordedCommand): string | undefined =>
-    /FileMode\]::(\w+)/.exec(command.script)?.[1]
+const fileWrites = (): RecordedCommand[] =>
+  commands.filter((command) => command.script.includes('OpenStandardInput'))
+const writtenPath = (command: RecordedCommand): string =>
+  /\$path = '((?:[^']|'')*)'/.exec(command.script)?.[1]?.replace(/''/g, "'") ?? ''
+const fileMode = (command: RecordedCommand): string | undefined =>
+  /FileMode\]::(\w+)/.exec(command.script)?.[1]
+const putLines = (): string[] =>
+  sftpBatches.flatMap((batch) => batch.script.split('\n').filter((line) => line.startsWith('put ')))
+const putDestination = (line: string): string => /put "(?:[^"]*)" "([^"]*)"/.exec(line)?.[1] ?? ''
+const putSource = (line: string): string => /put "([^"]*)"/.exec(line)?.[1] ?? ''
 
-  beforeEach(() => {
-    commands.length = 0
-    failAtSpawn = -1
-    waitForChannelCloseSpy.mockClear()
-    localDir = mkdtempSync(join(tmpdir(), 'orca-win-upload-'))
-    spawnSystemSshCommandMock.mockReset()
-    spawnSystemSshCommandMock.mockImplementation((_target: SshTarget, command: string) => {
-      const spawnIndex = spawnSystemSshCommandMock.mock.calls.length - 1
-      return createFakeChannel((channel) => {
-        commands.push({ script: decodePowerShellCommand(command), stdin: channel.written })
-        setImmediate(() =>
-          spawnIndex === failAtSpawn
-            ? channel.emit('close', 1, null)
-            : channel.emit('close', 0, null)
-        )
+/** Makes every sftp batch succeed, recording what it was asked to do. */
+function acceptSftp(): void {
+  runProcessMock.mockImplementation(
+    async (spec: { args: string[]; input: string; program: string }) => {
+      const script = spec.input
+      sftpBatches.push({ args: spec.args, script })
+      // Model the real client: `put` copies the local file, so read it while it still exists.
+      for (const line of script.split('\n').filter((entry) => entry.startsWith('put '))) {
+        await readFile(putSource(line))
+      }
+      return { code: 0, signal: null, stdout: '', stderr: '', timedOut: false }
+    }
+  )
+}
+
+/** Models a host whose sshd has no `Subsystem sftp` line. */
+function refuseSftp(): void {
+  runProcessMock.mockImplementation(async (spec: { args: string[]; input: string }) => {
+    sftpBatches.push({ args: spec.args, script: spec.input })
+    return {
+      code: 255,
+      signal: null,
+      stdout: '',
+      stderr: 'subsystem request failed on channel 0\nConnection closed',
+      timedOut: false
+    }
+  })
+}
+
+/** Models a host with no PowerShell 7, which cmd.exe reports as an unrecognized command. */
+function refusePwsh(): void {
+  spawnSystemSshCommandMock.mockImplementation((_target: SshTarget, command: string) => {
+    const spawnIndex = spawnSystemSshCommandMock.mock.calls.length - 1
+    const executable = command.split(' ')[0] ?? ''
+    return createFakeChannel((channel) => {
+      commands.push({
+        script: decodePowerShellCommand(command),
+        executable,
+        stdin: channel.written
+      })
+      setImmediate(() => {
+        if (executable === 'pwsh.exe') {
+          channel.stderr.write(
+            "'pwsh.exe' is not recognized as an internal or external command,\noperable program or batch file."
+          )
+          channel.emit('close', 9009, null)
+          return
+        }
+        channel.emit('close', spawnIndex === failAtSpawn ? 1 : 0, null)
       })
     })
   })
+}
 
-  afterEach(async () => {
-    await rm(localDir, { recursive: true, force: true })
+beforeEach(() => {
+  commands.length = 0
+  sftpBatches.length = 0
+  failAtSpawn = -1
+  clearWindowsRemoteWriteCapabilitiesForTests()
+  waitForChannelCloseSpy.mockClear()
+  localDir = mkdtempSync(join(tmpdir(), 'orca-win-upload-'))
+  process.env.ORCA_SYSTEM_SFTP_PATH = '/usr/bin/sftp'
+  runProcessMock.mockReset()
+  acceptSftp()
+  spawnSystemSshCommandMock.mockReset()
+  spawnSystemSshCommandMock.mockImplementation((_target: SshTarget, command: string) => {
+    const spawnIndex = spawnSystemSshCommandMock.mock.calls.length - 1
+    return createFakeChannel((channel) => {
+      commands.push({
+        script: decodePowerShellCommand(command),
+        executable: command.split(' ')[0] ?? '',
+        stdin: channel.written
+      })
+      setImmediate(() =>
+        spawnIndex === failAtSpawn ? channel.emit('close', 1, null) : channel.emit('close', 0, null)
+      )
+    })
   })
+})
 
-  it('never pushes a whole artifact bundle into one PowerShell stdin', async () => {
-    mkdirSync(join(localDir, 'node'), { recursive: true })
-    // Comfortably past the ~50KB point at which the reporter measured PowerShell 5.1 wedging.
-    writeFileSync(join(localDir, 'node', 'relay.js'), Buffer.alloc(600 * 1024, 0x61))
-    writeFileSync(join(localDir, 'index.js'), Buffer.alloc(300 * 1024, 0x62))
+afterEach(async () => {
+  delete process.env.ORCA_SYSTEM_SFTP_PATH
+  await rm(localDir, { recursive: true, force: true })
+})
 
-    await uploadDirectoryViaSystemSsh(target, localDir, remoteRoot, { hostPlatform })
-
-    const largest = Math.max(...commands.map((command) => command.stdin.length))
-    expect(largest).toBeLessThanOrEqual(WINDOWS_STDIN_WRITE_CHUNK_BYTES)
-    // The base64 + JSON envelope is gone entirely: nothing reads the bundle as one string.
-    expect(commands.some((command) => command.script.includes('FromBase64String'))).toBe(false)
-    // `[Console]::In` wedged at 50KB where the stream reader did not, so the mkdir batch — the one
-    // payload still read as a string — must use the reader the reporter measured surviving.
-    expect(commands.some((command) => command.script.includes('[Console]::In.ReadToEnd()'))).toBe(
-      false
-    )
-    expect(
-      commands.filter((command) => command.script.includes('StreamReader([Console]::'))
-    ).toHaveLength(1)
-  })
-
-  it('bounds the single-file upload too, which is the path large files take', async () => {
-    const contents = Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 3 + 11, 0x64)
+describe('Windows upload over sftp', () => {
+  it('moves the payload without any remote process reading a stdin', async () => {
+    const contents = Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 60 + 11, 0x64)
     const localPath = join(localDir, 'big.node')
     writeFileSync(localPath, contents)
 
     await uploadFileViaSystemSsh(target, localPath, `${remoteRoot}/big.node`, { hostPlatform })
 
-    const writes = fileWrites()
-    expect(writes).toHaveLength(4)
-    expect(Math.max(...writes.map((write) => write.stdin.length))).toBe(
-      WINDOWS_STDIN_WRITE_CHUNK_BYTES
-    )
-    expect(Buffer.concat(writes.map((write) => write.stdin)).equals(contents)).toBe(true)
-    // A wedged PowerShell never closes on its own, so no wait on this path may be unbounded.
-    expect(
-      waitForChannelCloseSpy.mock.calls.every((call) => call[2] === WINDOWS_STDIN_WRITE_TIMEOUT_MS)
-    ).toBe(true)
+    // The defect is a remote stdin read; the fix is that there is not one.
+    expect(fileWrites()).toHaveLength(0)
+    expect(putLines()).toHaveLength(1)
+    // One transfer, not 61 execs: the whole point of the change.
+    expect(sftpBatches).toHaveLength(1)
   })
 
-  it('writes every byte of every artifact across the chunked writes', async () => {
-    const contents = Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 2 + 17, 0x63)
-    writeFileSync(join(localDir, 'relay.js'), contents)
+  it('creates the parent chain and sends the payload in one round trip', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
 
-    await uploadDirectoryViaSystemSsh(target, localDir, remoteRoot, { hostPlatform })
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/a/b/relay.js`, {
+      hostPlatform
+    })
 
-    const writes = fileWrites()
-    expect(writes).toHaveLength(3)
-    expect(Buffer.concat(writes.map((write) => write.stdin)).equals(contents)).toBe(true)
-    // Only the first write creates the staging file; the rest must extend it or it is truncated.
-    expect(writes.map(fileMode)).toEqual(['Create', 'Append', 'Append'])
+    expect(sftpBatches).toHaveLength(1)
+    expect(sftpBatches[0]!.script.split('\n').filter(Boolean)).toEqual([
+      '-mkdir "/C:/Users"',
+      '-mkdir "/C:/Users/dev"',
+      '-mkdir "/C:/Users/dev/.orca-remote"',
+      '-mkdir "/C:/Users/dev/.orca-remote/a"',
+      '-mkdir "/C:/Users/dev/.orca-remote/a/b"',
+      expect.stringContaining('put ') as unknown as string
+    ])
+  })
+
+  it('addresses the destination in the drive-rooted namespace sftp exposes', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform
+    })
+
+    // A backslash destination silently writes a file named `C` and still exits 0, so the leading
+    // slash and forward separators are correctness, not style.
+    expect(putDestination(putLines()[0]!)).toMatch(
+      /^\/C:\/Users\/dev\/\.orca-remote\/relay\.js\.orca-partial-[0-9a-f]{12}$/
+    )
+  })
+
+  it('never lands a partial under the real name, and publishes by rename', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
+    const remotePath = `${remoteRoot}/relay.js`
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), remotePath, { hostPlatform })
+
+    expect(putDestination(putLines()[0]!)).not.toBe(`/C:${remotePath.slice(2)}`)
+    const publish = commands.at(-1)!
+    expect(publish.script).toContain('[System.IO.File]::Move($staging, $path)')
+    expect(publish.script).toContain('[System.IO.File]::Delete($path)')
+    // The publish reads the staged file, never a pipe, so it is safe on PowerShell 5.1.
+    expect(publish.script).not.toContain('OpenStandardInput')
+  })
+
+  it('gives every attempt its own staging name, so a retry cannot meet a predecessor lock', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform
+    })
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform
+    })
+
+    const [first, second] = putLines().map(putDestination)
+    expect(first).toContain(WINDOWS_STAGED_WRITE_SUFFIX)
+    // Losing contact is not evidence the previous writer died, so the name must not be reused.
+    expect(second).not.toBe(first)
+  })
+
+  it('enforces exclusive at the rename, where it is atomic', async () => {
+    writeFileSync(join(localDir, 'import.bin'), 'x')
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'import.bin'), `${remoteRoot}/import.bin`, {
+      hostPlatform,
+      exclusive: true
+    })
+
+    const publish = commands.at(-1)!
+    expect(publish.script).toContain('[System.IO.File]::Move($staging, $path)')
+    expect(publish.script).not.toContain('[System.IO.File]::Delete($path)')
+  })
+
+  it('appends by concatenating the staged file, not by piping bytes to the remote', async () => {
+    await writeBufferViaSystemSsh(target, `${remoteRoot}/log.bin`, Buffer.from('tail'), {
+      hostPlatform,
+      append: true
+    })
+
+    expect(fileWrites()).toHaveLength(0)
+    const publish = commands.at(-1)!
+    expect(publish.script).toContain('FileMode]::Append')
+    expect(publish.script).toContain('$in.CopyTo($out)')
+    expect(publish.script).toContain('[System.IO.File]::Delete($staging)')
   })
 
   it('still creates an empty artifact on the host', async () => {
@@ -175,80 +312,232 @@ describe('Windows upload stdin framing', () => {
 
     await uploadDirectoryViaSystemSsh(target, localDir, remoteRoot, { hostPlatform })
 
-    expect(fileWrites().map(writtenPath)).toEqual([`${remoteRoot}/empty.txt`])
-    expect(fileWrites()[0].stdin).toHaveLength(0)
-    expect(fileMode(fileWrites()[0])).toBe('Create')
+    expect(putLines()).toHaveLength(1)
+    expect(commands.at(-1)!.script).toContain('[System.IO.File]::Move($staging, $path)')
   })
 
-  it('lands a multi-chunk write on a staging path and publishes it by rename', async () => {
-    const remotePath = `${remoteRoot}/relay.js`
-    writeFileSync(join(localDir, 'relay.js'), Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES + 1))
+  it('writes a buffer through a 0600 temp file that does not outlive the transfer', async () => {
+    const seen: { path: string; contents: Buffer }[] = []
+    runProcessMock.mockImplementation(async (spec: { args: string[]; input: string }) => {
+      sftpBatches.push({ args: spec.args, script: spec.input })
+      for (const line of spec.input.split('\n').filter((entry) => entry.startsWith('put '))) {
+        const path = putSource(line)
+        seen.push({ path, contents: await readFile(path) })
+      }
+      return { code: 0, signal: null, stdout: '', stderr: '', timedOut: false }
+    })
+
+    await writeBufferViaSystemSsh(target, `${remoteRoot}/version`, Buffer.from('1.2.3'), {
+      hostPlatform
+    })
+
+    expect(seen).toHaveLength(1)
+    expect(seen[0]!.contents.toString()).toBe('1.2.3')
+    await expect(readFile(seen[0]!.path)).rejects.toThrow()
+  })
+
+  it('creates upload directories over sftp rather than a PowerShell stdin batch', async () => {
+    mkdirSync(join(localDir, 'node'), { recursive: true })
+    writeFileSync(join(localDir, 'node', 'relay.js'), 'x')
 
     await uploadDirectoryViaSystemSsh(target, localDir, remoteRoot, { hostPlatform })
 
-    // Nothing touches the real name until every byte is on the host.
-    expect(fileWrites().map(writtenPath)).toEqual([
-      `${remotePath}${WINDOWS_STAGED_WRITE_SUFFIX}`,
-      `${remotePath}${WINDOWS_STAGED_WRITE_SUFFIX}`
-    ])
-    const publish = commands.at(-1)!
-    expect(publish.script).toContain('[System.IO.File]::Move($staging, $path)')
-    expect(publish.script).toContain('[System.IO.File]::Delete($path)')
+    expect(commands.some((command) => command.script.includes('StreamReader([Console]::'))).toBe(
+      false
+    )
+    expect(sftpBatches[0]!.script).toContain('-mkdir "/C:/Users/dev/.orca-remote"')
+  })
+
+  it('sweeps the staged bytes when the publish is the thing that fails', async () => {
+    writeFileSync(join(localDir, 'import.bin'), 'x')
+    // An exclusive conflict is the ordinary way to get here: the payload is on the host, and the
+    // rename that would have given it a name refuses.
+    spawnSystemSshCommandMock.mockImplementation((_target: SshTarget, command: string) => {
+      const script = decodePowerShellCommand(command)
+      return createFakeChannel((channel) => {
+        commands.push({ script, executable: command.split(' ')[0] ?? '', stdin: channel.written })
+        const failed = script.includes('::Move($staging, $path)')
+        setImmediate(() => channel.emit('close', failed ? 1 : 0, null))
+      })
+    })
+
+    await expect(
+      uploadFileViaSystemSsh(target, join(localDir, 'import.bin'), `${remoteRoot}/import.bin`, {
+        hostPlatform,
+        exclusive: true
+      })
+    ).rejects.toThrow()
+
+    const sweep = commands.at(-1)!
+    expect(sweep.script).toContain('[System.IO.File]::Delete($staging)')
+    // Tolerated, not asserted: the previous writer may still hold the file, and losing contact is
+    // not evidence it died.
+    expect(sweep.script).toContain('$ErrorActionPreference = "SilentlyContinue"')
+  })
+
+  it('reports a cancelled transfer as an abort, not as a failed one', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
+    const controller = new AbortController()
+    // runProcess reports the kill as a non-zero exit rather than throwing, so without checking the
+    // signal first a user pressing cancel is indistinguishable from the transfer genuinely failing.
+    runProcessMock.mockImplementation(async (spec: { args: string[]; input: string }) => {
+      sftpBatches.push({ args: spec.args, script: spec.input })
+      controller.abort()
+      return { code: 255, signal: 'SIGTERM', stdout: '', stderr: '', timedOut: false }
+    })
+
+    let error: Error | undefined
+    try {
+      await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+        hostPlatform,
+        signal: controller.signal
+      })
+    } catch (thrown) {
+      error = thrown as Error
+    }
+
+    expect(error?.name).toBe('AbortError')
+    expect(error?.message).not.toContain('sftp batch failed')
+    // A cancel is also not evidence about the host, so it must not send later writes to the slow
+    // path, and must not fall through to the defective reader now.
+    expect(getWindowsRemoteWriteCapabilities(target).shouldTry('sftp-subsystem')).toBe(true)
+    expect(fileWrites()).toHaveLength(0)
+  })
+
+  it('translates the ssh argument list rather than passing it to a client that reads it differently', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform,
+      disableControlMaster: true
+    })
+
+    const args = sftpBatches[0]!.args
+    // sftp's `-T` does not exist, its `-p` preserves mtime, and its `-S` names a program to run.
+    expect(args).not.toContain('-T')
+    expect(args).not.toContain('-p')
+    expect(args).not.toContain('-S')
+    expect(args).toContain('ControlPath=none')
+    expect(args).toContain('ServerAliveInterval=15')
+  })
+})
+
+describe('Windows upload on a host with no sftp subsystem', () => {
+  beforeEach(() => {
+    refuseSftp()
+  })
+
+  it('falls back rather than failing the transfer', async () => {
+    const contents = Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES + 5, 0x61)
+    writeFileSync(join(localDir, 'relay.js'), contents)
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform
+    })
+
+    expect(Buffer.concat(fileWrites().map((write) => write.stdin)).equals(contents)).toBe(true)
+  })
+
+  it('remembers the refusal, so a multi-file upload probes once', async () => {
+    writeFileSync(join(localDir, 'a.js'), 'a')
+    writeFileSync(join(localDir, 'b.js'), 'b')
+    writeFileSync(join(localDir, 'c.js'), 'c')
+
+    await uploadDirectoryViaSystemSsh(target, localDir, remoteRoot, { hostPlatform })
+
+    // One refusal is enough; re-probing per file is a wasted round trip on every file.
+    expect(sftpBatches).toHaveLength(1)
+  })
+
+  it('does not spend a sweep round trip when sftp declined before moving any bytes', async () => {
+    writeFileSync(join(localDir, 'relay.js'), 'x')
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform
+    })
+
+    // A refused subsystem staged nothing, so there is nothing to delete — and on a host without
+    // sftp that sweep would otherwise be paid on every single write.
+    expect(commands.some((command) => command.script.includes('Delete($staging)'))).toBe(false)
+  })
+
+  it('prefers PowerShell 7, which reads a redirected stdin correctly', async () => {
+    writeFileSync(join(localDir, 'relay.js'), Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 3))
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+      hostPlatform
+    })
+
+    expect(fileWrites().map((write) => write.executable)).toEqual(['pwsh.exe'])
+    // PowerShell 7 took 2MB through one exec when measured, so chunking it buys nothing.
+    expect(fileWrites()[0]!.stdin).toHaveLength(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 3)
+  })
+
+  it('bounds every write when only Windows PowerShell 5.1 is available', async () => {
+    refusePwsh()
+    const contents = Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 3 + 11, 0x64)
+    writeFileSync(join(localDir, 'big.node'), contents)
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'big.node'), `${remoteRoot}/big.node`, {
+      hostPlatform
+    })
+
+    const writes = fileWrites().filter((write) => write.executable === 'powershell.exe')
+    expect(writes).toHaveLength(4)
+    expect(Math.max(...writes.map((write) => write.stdin.length))).toBe(
+      WINDOWS_STDIN_WRITE_CHUNK_BYTES
+    )
+    expect(Buffer.concat(writes.map((write) => write.stdin)).equals(contents)).toBe(true)
+    expect(writes.map(fileMode)).toEqual(['Create', 'Append', 'Append', 'Append'])
+    // A wedged PowerShell never closes on its own, so no wait on this path may be unbounded.
+    expect(
+      waitForChannelCloseSpy.mock.calls.every((call) => call[2] === WINDOWS_STDIN_WRITE_TIMEOUT_MS)
+    ).toBe(true)
+  })
+
+  it('remembers that PowerShell 7 is absent instead of re-probing per chunk', async () => {
+    refusePwsh()
+    writeFileSync(join(localDir, 'big.node'), Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 3))
+
+    await uploadFileViaSystemSsh(target, join(localDir, 'big.node'), `${remoteRoot}/big.node`, {
+      hostPlatform
+    })
+
+    expect(fileWrites().filter((write) => write.executable === 'pwsh.exe')).toHaveLength(1)
   })
 
   it('leaves no truncated file under the real name when a chunk fails mid-file', async () => {
     writeFileSync(join(localDir, 'relay.js'), Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES * 3))
-    // Spawns: 0 = mkdir batch, 1..3 = chunk writes. Fail the second chunk.
-    failAtSpawn = 2
+    // Spawn 0 is the pwsh write; fail it and every retry beneath it.
+    failAtSpawn = 0
 
     await expect(
-      uploadDirectoryViaSystemSsh(target, localDir, remoteRoot, { hostPlatform })
+      uploadFileViaSystemSsh(target, join(localDir, 'relay.js'), `${remoteRoot}/relay.js`, {
+        hostPlatform
+      })
     ).rejects.toThrow()
 
     expect(fileWrites().map(writtenPath)).not.toContain(`${remoteRoot}/relay.js`)
     expect(commands.some((command) => command.script.includes('::Move('))).toBe(false)
   })
+})
 
-  it('enforces exclusive once at the rename, so a retry is not blocked by its own leftovers', async () => {
-    const localPath = join(localDir, 'import.bin')
-    writeFileSync(localPath, Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES + 1))
+describe('last-resort Windows PowerShell failure reporting', () => {
+  it('names the host limitation and its remedy, not just the timeout', () => {
+    const timeout = new Error('write C:/x at offset 0 timed out after 60000ms with no response')
 
-    await uploadFileViaSystemSsh(target, localPath, `${remoteRoot}/import.bin`, {
-      hostPlatform,
-      exclusive: true
-    })
+    const explained = explainWindowsPowerShellStdinFailure(timeout) as Error
 
-    // CreateNew on chunk one would fail against a leftover staging file from a failed attempt;
-    // `File::Move` raising on an existing destination is what carries the exclusive contract.
-    expect(fileWrites().map(fileMode)).toEqual(['Create', 'Append'])
-    const publish = commands.at(-1)!
-    expect(publish.script).toContain('[System.IO.File]::Move($staging, $path)')
-    expect(publish.script).not.toContain('[System.IO.File]::Delete($path)')
+    // "timed out" alone sends the user to retry a network they cannot fix; the fix is host-side.
+    expect(explained.message).toContain('Windows PowerShell 5.1')
+    expect(explained.message).toContain('Subsystem sftp sftp-server.exe')
+    expect(explained.cause).toBe(timeout)
   })
 
-  it('keeps a single-chunk write on the destination, with the caller mode intact', async () => {
-    await writeBufferViaSystemSsh(target, `${remoteRoot}/version`, Buffer.from('1.2.3'), {
-      hostPlatform,
-      exclusive: true
-    })
+  it('leaves a real failure alone, so a permission error is not reported as a host limitation', () => {
+    const denied = new Error('write C:/x at offset 0 failed (exit 1): Access to the path is denied')
 
-    expect(fileWrites()).toHaveLength(1)
-    expect(writtenPath(fileWrites()[0])).toBe(`${remoteRoot}/version`)
-    expect(fileMode(fileWrites()[0])).toBe('CreateNew')
-    expect(commands.some((command) => command.script.includes('::Move('))).toBe(false)
-  })
-
-  it('appends onto the destination rather than staging, since append cannot be staged', async () => {
-    const remotePath = `${remoteRoot}/log.bin`
-    await writeBufferViaSystemSsh(
-      target,
-      remotePath,
-      Buffer.alloc(WINDOWS_STDIN_WRITE_CHUNK_BYTES + 1),
-      { hostPlatform, append: true }
-    )
-
-    expect(fileWrites().map(writtenPath)).toEqual([remotePath, remotePath])
-    expect(fileWrites().map(fileMode)).toEqual(['Append', 'Append'])
+    expect(explainWindowsPowerShellStdinFailure(denied)).toBe(denied)
   })
 })
 
diff --git a/src/main/ssh/system-ssh-windows-write-capabilities.test.ts b/src/main/ssh/system-ssh-windows-write-capabilities.test.ts
new file mode 100644
index 00000000000..ad723d592b0
--- /dev/null
+++ b/src/main/ssh/system-ssh-windows-write-capabilities.test.ts
@@ -0,0 +1,79 @@
+/**
+ * Whether a Windows host has an sftp subsystem is a fact about that host, so the cache is keyed by
+ * the endpoint that executes rather than by Orca's target id — otherwise a hardened host is
+ * re-probed once per file, and two targets pointing at one machine learn the same fact twice.
+ */
+import { afterEach, describe, expect, it } from 'vitest'
+import type { SshTarget } from '../../shared/ssh-types'
+import {
+  clearWindowsRemoteWriteCapabilitiesForTests,
+  getWindowsRemoteWriteCapabilities,
+  getWindowsRemoteWriteExecutionHostKey
+} from './system-ssh-windows-write-capabilities'
+
+const asTarget = (fields: Partial<SshTarget>): SshTarget => fields as SshTarget
+
+afterEach(() => {
+  clearWindowsRemoteWriteCapabilitiesForTests()
+})
+
+describe('getWindowsRemoteWriteExecutionHostKey', () => {
+  it('gives two targets on one endpoint the same key', () => {
+    const first = asTarget({ id: 'a', host: 'win.example', username: 'dev', port: 22 })
+    const second = asTarget({ id: 'b', host: 'win.example', username: 'dev', port: 22 })
+
+    // A target re-created under a new id has not changed what the host supports.
+    expect(getWindowsRemoteWriteExecutionHostKey(first)).toBe(
+      getWindowsRemoteWriteExecutionHostKey(second)
+    )
+  })
+
+  it('separates hosts, ports and users', () => {
+    const base = { id: 'a', host: 'win.example', username: 'dev', port: 22 }
+    const keys = [
+      asTarget(base),
+      asTarget({ ...base, host: 'other.example' }),
+      asTarget({ ...base, port: 2222 }),
+      asTarget({ ...base, username: 'ops' })
+    ].map(getWindowsRemoteWriteExecutionHostKey)
+
+    expect(new Set(keys).size).toBe(4)
+  })
+
+  it('keys a config alias by the alias, since ssh_config decides where it lands', () => {
+    const alias = asTarget({ id: 'a', host: 'stale.example', configHost: 'winbox' })
+
+    expect(getWindowsRemoteWriteExecutionHostKey(alias)).toBe('config:winbox')
+  })
+})
+
+describe('getWindowsRemoteWriteCapabilities', () => {
+  it('shares one cache across targets that reach the same host', () => {
+    const first = asTarget({ id: 'a', host: 'win.example', username: 'dev', port: 22 })
+    const second = asTarget({ id: 'b', host: 'win.example', username: 'dev', port: 22 })
+
+    getWindowsRemoteWriteCapabilities(first).rememberUnsupported('sftp-subsystem')
+
+    expect(getWindowsRemoteWriteCapabilities(second).shouldTry('sftp-subsystem')).toBe(false)
+  })
+
+  it('does not let one host answer for another', () => {
+    const hardened = asTarget({ id: 'a', host: 'hardened.example', username: 'dev', port: 22 })
+    const ordinary = asTarget({ id: 'b', host: 'ordinary.example', username: 'dev', port: 22 })
+
+    getWindowsRemoteWriteCapabilities(hardened).rememberUnsupported('sftp-subsystem')
+
+    expect(getWindowsRemoteWriteCapabilities(ordinary).shouldTry('sftp-subsystem')).toBe(true)
+  })
+
+  it('keeps the two capabilities independent', () => {
+    const target = asTarget({ id: 'a', host: 'win.example', username: 'dev', port: 22 })
+    const capabilities = getWindowsRemoteWriteCapabilities(target)
+
+    capabilities.rememberUnsupported('pwsh')
+
+    // No PowerShell 7 says nothing about whether the host will serve sftp.
+    expect(capabilities.shouldTry('sftp-subsystem')).toBe(true)
+    expect(capabilities.shouldTry('pwsh')).toBe(false)
+  })
+})
diff --git a/src/main/ssh/system-ssh-windows-write-capabilities.ts b/src/main/ssh/system-ssh-windows-write-capabilities.ts
new file mode 100644
index 00000000000..dcd03f19807
--- /dev/null
+++ b/src/main/ssh/system-ssh-windows-write-capabilities.ts
@@ -0,0 +1,52 @@
+import type { SshTarget } from '../../shared/ssh-types'
+import { CapabilityProbeCache } from '../../shared/capability-probe-cache'
+
+/**
+ * Whether a Windows host can take a file write over the sftp subsystem, and whether it has a
+ * PowerShell 7 to fall back to. Both are host facts, so they are cached per execution host rather
+ * than per transfer — a hardened host with `Subsystem sftp` removed must not be re-probed on every
+ * file of a multi-file upload.
+ */
+export type WindowsRemoteWriteCapability = 'sftp-subsystem' | 'pwsh'
+
+// Why re-probe at all: an admin can enable the subsystem, or install PowerShell 7, without the
+// user restarting Orca. Long enough that a hardened host costs one failed probe per half hour.
+export const WINDOWS_WRITE_CAPABILITY_RETRY_INTERVAL_MS = 30 * 60_000
+
+const capabilitiesByExecutionHost = new Map<
+  string,
+  CapabilityProbeCache<WindowsRemoteWriteCapability>
+>()
+
+/**
+ * Keyed by the endpoint that executes, not by target id: two Orca targets pointing at one host
+ * describe the same sshd, and a target re-created under a new id has not changed what that host
+ * supports. A config alias is its own key because ssh_config, not Orca, resolves where it lands.
+ */
+export function getWindowsRemoteWriteExecutionHostKey(target: SshTarget): string {
+  if (target.configHost) {
+    return `config:${target.configHost}`
+  }
+  const port = target.port ?? 22
+  return target.username
+    ? `host:${target.username}@${target.host}:${port}`
+    : `host:${target.host}:${port}`
+}
+
+export function getWindowsRemoteWriteCapabilities(
+  target: SshTarget
+): CapabilityProbeCache<WindowsRemoteWriteCapability> {
+  const key = getWindowsRemoteWriteExecutionHostKey(target)
+  let cache = capabilitiesByExecutionHost.get(key)
+  if (!cache) {
+    cache = new CapabilityProbeCache<WindowsRemoteWriteCapability>(
+      WINDOWS_WRITE_CAPABILITY_RETRY_INTERVAL_MS
+    )
+    capabilitiesByExecutionHost.set(key, cache)
+  }
+  return cache
+}
+
+export function clearWindowsRemoteWriteCapabilitiesForTests(): void {
+  capabilitiesByExecutionHost.clear()
+}
diff --git a/src/main/ssh/system-ssh-windows-write-strategy.ts b/src/main/ssh/system-ssh-windows-write-strategy.ts
new file mode 100644
index 00000000000..7df9f800925
--- /dev/null
+++ b/src/main/ssh/system-ssh-windows-write-strategy.ts
@@ -0,0 +1,301 @@
+import type { SshTarget } from '../../shared/ssh-types'
+import { getSystemSshBuildArgsFromOperationOptions } from './system-ssh-args'
+import { spawnSystemSshCommand } from './system-ssh-command'
+import {
+  awaitWithSystemSshAbort,
+  throwIfAborted,
+  waitForChannelClose
+} from './system-ssh-operation-lifecycle'
+import { isSftpUnavailableError, runSftpBatch } from './system-ssh-sftp-transfer'
+import { quoteSftpBatchArgument, toSftpRemotePath } from './system-ssh-sftp-path'
+import { getWindowsRemoteWriteCapabilities } from './system-ssh-windows-write-capabilities'
+import {
+  makeWindowsDiscardStagedFileCommand,
+  makeWindowsPublishStagedFileCommand,
+  makeWindowsStagingPath,
+  makeWindowsWriteFileCommand,
+  windowsRemoteAncestorDirectories,
+  type WindowsPublishMode
+} from './system-ssh-windows-file-write'
+
+/** No Windows stdin write should ever outlive this; a wedged PowerShell never closes on its own. */
+export const WINDOWS_STDIN_WRITE_TIMEOUT_MS = 60_000
+
+/**
+ * Bound on one stdin write for the last-resort Windows PowerShell 5.1 path.
+ *
+ * Measured on Windows 11 26200 / OpenSSH 10.0p2: a 32KB write still hangs 15 times in 120 under
+ * load, and no smaller value removes the risk. The defect is per blocking read, not per byte, so
+ * shrinking the chunk trades one risky read for more execs that each carry their own. This is a
+ * damage bound on a path known to be unreliable, not a safe size.
+ */
+export const WINDOWS_STDIN_WRITE_CHUNK_BYTES = 32 * 1024
+
+export type WindowsWriteOptions = Parameters<
+  typeof getSystemSshBuildArgsFromOperationOptions
+>[0] & {
+  signal?: AbortSignal
+  append?: boolean
+  exclusive?: boolean
+}
+
+/** Bytes to write, plus a way to present them to sftp, which can only send a local file. */
+export type WindowsWriteSource = {
+  totalBytes: number
+  readChunk: (offset: number, maxBytes: number) => Promise<Buffer>
+  withLocalFile: <T>(send: (localPath: string) => Promise<T>) => Promise<T>
+}
+
+function publishMode(options: WindowsWriteOptions): WindowsPublishMode {
+  return options.append ? 'append' : options.exclusive === true ? 'exclusive' : 'create'
+}
+
+/**
+ * Writes one file to a Windows host, preferring transports that do not push bytes through a remote
+ * PowerShell's stdin.
+ *
+ * Order, and why: sftp carries the whole payload in one transfer and never has a remote process
+ * read a pipe. Measured on Windows 11 / OpenSSH 10.0p2: 1.9MB in a median 315ms over sftp against
+ * 0 of 6 completions on the chunked path, whose best case was ~62 execs at ~350ms each. PowerShell
+ * 7 reads a redirected stdin correctly but is not installed by default. Windows PowerShell 5.1 is
+ * always present and is the defective reader, so it is last and it is bounded.
+ *
+ * Every transport stages under a unique name and publishes by rename, so no partial write is ever
+ * visible under the real name and no retry inherits a predecessor's lock.
+ */
+export async function writeWindowsRemoteFile(
+  target: SshTarget,
+  remotePath: string,
+  source: WindowsWriteSource,
+  options: WindowsWriteOptions
+): Promise<void> {
+  throwIfAborted(options.signal)
+  const capabilities = getWindowsRemoteWriteCapabilities(target)
+  await capabilities.runWithFallback(
+    'sftp-subsystem',
+    () => writeViaSftp(target, remotePath, source, options),
+    () => writeViaRemoteStdin(target, remotePath, source, options),
+    isSftpUnavailableError
+  )
+}
+
+/**
+ * Stages under a name nothing else can own, publishes it, and sweeps the staging file if either
+ * step fails.
+ *
+ * Shared by both transports so the cleanup contract cannot drift between them: a failed publish —
+ * an exclusive conflict is the ordinary case — leaves bytes on the host that no longer have a
+ * purpose, and the sweep is what stops them accumulating.
+ */
+async function stageThenPublish(
+  target: SshTarget,
+  remotePath: string,
+  options: WindowsWriteOptions,
+  stage: (stagingPath: string) => Promise<void>,
+  nothingStaged: (error: unknown) => boolean = () => false
+): Promise<void> {
+  const stagingPath = makeWindowsStagingPath(remotePath)
+  try {
+    await stage(stagingPath)
+    await publishStagedWrite(target, stagingPath, remotePath, options)
+  } catch (error) {
+    // A transport that declined before it moved any bytes has nothing to sweep, and sweeping
+    // anyway would spend a round trip on every write to a host that has no sftp subsystem.
+    if (!nothingStaged(error)) {
+      await discardStagedWrite(target, stagingPath, options)
+    }
+    throw error
+  }
+}
+
+function writeViaSftp(
+  target: SshTarget,
+  remotePath: string,
+  source: WindowsWriteSource,
+  options: WindowsWriteOptions
+): Promise<void> {
+  const mkdirs = windowsRemoteAncestorDirectories(remotePath).map(
+    (directory) => `-mkdir ${quoteSftpBatchArgument(toSftpRemotePath(directory))}`
+  )
+  return stageThenPublish(
+    target,
+    remotePath,
+    options,
+    (stagingPath) =>
+      source.withLocalFile((localPath) =>
+        // One round trip: the parent chain and the payload travel in the same batch.
+        runSftpBatch(
+          target,
+          [
+            ...mkdirs,
+            `put ${quoteSftpBatchArgument(localPath)} ${quoteSftpBatchArgument(toSftpRemotePath(stagingPath))}`
+          ],
+          options
+        )
+      ),
+    isSftpUnavailableError
+  )
+}
+
+function writeViaRemoteStdin(
+  target: SshTarget,
+  remotePath: string,
+  source: WindowsWriteSource,
+  options: WindowsWriteOptions
+): Promise<void> {
+  const capabilities = getWindowsRemoteWriteCapabilities(target)
+  return stageThenPublish(target, remotePath, options, (stagingPath) =>
+    capabilities.runWithFallback(
+      'pwsh',
+      () => writeStdinChunks(target, stagingPath, source, options, 'pwsh.exe'),
+      () => writeStdinChunks(target, stagingPath, source, options, 'powershell.exe'),
+      isPwshUnavailableError
+    )
+  )
+}
+
+/**
+ * PowerShell 7 takes the whole payload in one exec — measured at 2MB — so only the 5.1 path pays
+ * for chunking, and only because a bounded write is the most that path can be trusted with.
+ */
+async function writeStdinChunks(
+  target: SshTarget,
+  stagingPath: string,
+  source: WindowsWriteSource,
+  options: WindowsWriteOptions,
+  executable: 'powershell.exe' | 'pwsh.exe'
+): Promise<void> {
+  const chunkBytes =
+    executable === 'pwsh.exe' ? Math.max(source.totalBytes, 1) : WINDOWS_STDIN_WRITE_CHUNK_BYTES
+  let offset = 0
+  // An empty write still has to run: it is what creates the staged file.
+  do {
+    const chunk = await source.readChunk(offset, chunkBytes)
+    if (chunk.length === 0 && offset < source.totalBytes) {
+      throw new Error(`Source ran short during upload of ${stagingPath}`)
+    }
+    await writeOneStdinChunk(
+      target,
+      stagingPath,
+      chunk,
+      { ...options, append: offset > 0, exclusive: false },
+      offset,
+      executable
+    )
+    offset += chunk.length
+  } while (offset < source.totalBytes)
+}
+
+async function writeOneStdinChunk(
+  target: SshTarget,
+  stagingPath: string,
+  chunk: Buffer,
+  options: WindowsWriteOptions,
+  offset: number,
+  executable: 'powershell.exe' | 'pwsh.exe'
+): Promise<void> {
+  throwIfAborted(options.signal)
+  const channel = spawnSystemSshCommand(
+    target,
+    makeWindowsWriteFileCommand(stagingPath, {
+      append: options.append,
+      exclusive: options.exclusive,
+      executable
+    }),
+    { wrapCommand: false, ...getSystemSshBuildArgsFromOperationOptions(options) }
+  )
+  const closePromise = awaitWithSystemSshAbort(
+    options.signal,
+    () => channel.close(),
+    waitForChannelClose(
+      channel,
+      `write ${stagingPath} at offset ${offset}`,
+      WINDOWS_STDIN_WRITE_TIMEOUT_MS
+    )
+  ).catch((error: unknown) => {
+    throw executable === 'powershell.exe' ? explainWindowsPowerShellStdinFailure(error) : error
+  })
+  if (!options.signal?.aborted) {
+    channel.stdin.end(chunk)
+  }
+  await closePromise
+}
+
+/**
+ * Names the cause on the one path that can hang, so the failure is not just "timed out".
+ *
+ * A user seeing this needs to know it is a host limitation with a host-side remedy, not a network
+ * fault they should retry into.
+ */
+export function explainWindowsPowerShellStdinFailure(error: unknown): unknown {
+  const message = error instanceof Error ? error.message : String(error)
+  if (!/timed out/i.test(message)) {
+    return error
+  }
+  return new Error(
+    `${message}\nWindows PowerShell 5.1 can lose a redirected stdin permanently when a read finds it momentarily empty, so this write cannot be made reliable from the client. Enable the sftp subsystem on the host (sshd_config: "Subsystem sftp sftp-server.exe"), or install PowerShell 7, and Orca will use it automatically.`,
+    { cause: error instanceof Error ? error : undefined }
+  )
+}
+
+function isPwshUnavailableError(error: unknown): boolean {
+  const message = error instanceof Error ? error.message : String(error)
+  // cmd.exe's "not recognized" and sshd's exit 9009 both mean "no pwsh here". A timeout does not:
+  // that is the stdin defect, and PowerShell 7 does not have it, so it must not be cached as absent.
+  return /is not recognized as an internal or external command|9009|CommandNotFoundException/i.test(
+    message
+  )
+}
+
+async function publishStagedWrite(
+  target: SshTarget,
+  stagingPath: string,
+  remotePath: string,
+  options: WindowsWriteOptions
+): Promise<void> {
+  await runWindowsCommandWithoutStdin(
+    target,
+    makeWindowsPublishStagedFileCommand(stagingPath, remotePath, publishMode(options)),
+    `publish ${remotePath}`,
+    options
+  )
+}
+
+async function discardStagedWrite(
+  target: SshTarget,
+  stagingPath: string,
+  options: WindowsWriteOptions
+): Promise<void> {
+  try {
+    await runWindowsCommandWithoutStdin(
+      target,
+      makeWindowsDiscardStagedFileCommand(stagingPath),
+      `discard ${stagingPath}`,
+      { ...options, signal: undefined }
+    )
+  } catch {
+    // Housekeeping only. The staging name is unique, so a leftover blocks nothing, and a failure
+    // here says nothing about whether the abandoned writer is still alive.
+  }
+}
+
+function runWindowsCommandWithoutStdin(
+  target: SshTarget,
+  command: string,
+  label: string,
+  options: WindowsWriteOptions
+): Promise<void> {
+  const channel = spawnSystemSshCommand(target, command, {
+    wrapCommand: false,
+    ...getSystemSshBuildArgsFromOperationOptions(options)
+  })
+  const closePromise = awaitWithSystemSshAbort(
+    options.signal,
+    () => channel.close(),
+    waitForChannelClose(channel, label, WINDOWS_STDIN_WRITE_TIMEOUT_MS)
+  )
+  if (!options.signal?.aborted) {
+    channel.stdin.end()
+  }
+  return closePromise
+}